Frequency selective passive component networks for implantable leads of active implantable medical devices utilizing an energy dissipating surface

ABSTRACT

Decoupling circuits are provided which transfer energy induced from an MRI pulsed RF field to an energy dissipating surface. This is accomplished through broadband filtering or by resonant filtering. In a passive component network for an implantable leadwire of an active implantable medical device, a frequency selective energy diversion circuit is provided for diverting high-frequency energy away from a leadwire electrode to a point or an area spaced from the electrode, for dissipation of high-frequency energy.

BACKGROUND OF THE INVENTION

This invention generally relates to the problem of energy induced intoimplanted leads during medical diagnostic procedures such as magneticresonant imaging (MRI). Specifically, the radio frequency (RF) pulsedfield of MRI can couple to an implanted lead in such a way thatelectromagnetic forces (EMFs) are induced in the lead. The amount ofenergy that is induced is related to a number of complex factors, but ingeneral, is dependent upon the local electric field that is tangent tolead and the integral of the tangential electric field strength alongthe lead. In certain situations, these EMFs can cause currents to flowinto distal electrodes or in the electrode interface with body tissue.It has been documented that when this current becomes excessive, thatoverheating of said electrode or overheating of the associated interfacewith body tissue can occur. There have been cases of damage to such bodytissue which has resulted in loss of capture of cardiac pacemakingpulses, tissue damage, severe enough to result in brain damage ormultiple amputations, and the like. The present invention relatesgenerally to methods of redirecting said energy to other locations otherthan a distal tip electrode-to-tissue interface. The redirection of thisRF energy is generally done by use of frequency selective devices, suchas inductors, capacitors and filtered networks. In general, this isaccomplished through frequency selective low pass filters or seriesresonant LC trap filters wherein the RF energy can be redirected toanother surface or is converted to heat. These implantable lead systemsare generally associated with active implantable medical devices(AIMDs), such as cardiac pacemakers, cardioverter defibrillators,neurostimulators and the like. Implantable leads can also be associatedwith external devices, such as external pacemakers, externally wornneurostimulators (such as pain control spinal cord stimulators) and thelike.

Compatibility of cardiac pacemakers, implantable defibrillators andother types of active implantable medical devices with magneticresonance imaging (MRI) and other types of hospital diagnostic equipmenthas become a major issue. If one goes to the websites of the majorcardiac pacemaker manufacturers in the United States, which include St.Jude Medical, Medtronic and Boston Scientific (formerly Guidant), onewill see that the use of MRI is generally contra-indicated withpacemakers and implantable defibrillators. See also:

-   (1) Safety Aspects of Cardiac Pacemakers in Magnetic Resonance    Imaging”, a dissertation submitted to the Swiss Federal Institute of    Technology Zurich presented by Roger Christoph Luchinger, Zurich    2002;-   (2) “1. Dielectric Properties of Biological Tissues: Literature    Survey”, by C. Gabriel, S. Gabriel and E. Cortout;-   (3) “II. Dielectric Properties of Biological Tissues: Measurements    and the Frequency Range 0 Hz to 20 GHz”, by S. Gabriel, R. W. Lau    and C. Gabriel;-   (4) “III. Dielectric Properties of Biological Tissues: Parametric    Models for the Dielectric Spectrum of Tissues”, by S. Gabriel, R. W.    Lau and C. Gabriel; and-   (5) “Advanced Engineering Electromagnetics, C. A. Balanis, Wiley,    1989;-   (6) Systems and Methods for Magnetic-Resonance-Guided Interventional    Procedures, Patent Application Publication US 2003/0050557, Susil    and Halperin et. al, published Mar. 13, 2003;-   (7) Multifunctional Interventional Devices for MRI: A Combined    Electrophysiology/MRI Catheter, by, Robert C. Susil, Henry R.    Halperin, Christopher J. Yeung, Albert C. Lardo and Ergin Atalar,    MRI in Medicine, 2002; and-   (8) Multifunctional Interventional Devices for Use in MRI, U.S.    Patent Application Ser. No. 60/283,725, filed Apr. 13, 2001.

The contents of the foregoing are all incorporated herein by reference.

However, an extensive review of the literature indicates that MRI isindeed often used with pacemaker, neurostimulator and other activeimplantable medical device (AIMD) patients. The safety and feasibilityof MRI in patients with cardiac pacemakers is an issue of gainingsignificance. The effects of MRI on patients' pacemaker systems haveonly been analyzed retrospectively in some case reports. There are anumber of papers that indicate that MRI on new generation pacemakers canbe conducted up to 0.5 Tesla (T). MRI is one of medicine's most valuablediagnostic tools. MRI is, of course, extensively used for imaging, butis also used for interventional medicine (surgery). In addition, MRI isused in real time to guide ablation catheters, neurostimulator tips,deep brain probes and the like. An absolute contra-indication forpacemaker patients means that pacemaker and implantable cardioverterdefibrillator (ICD) wearers are excluded from MRI. This is particularlytrue of scans of the thorax and abdominal areas. Because of MRI'sincredible value as a diagnostic tool for imaging organs and other bodytissues, many physicians simply take the risk and go ahead and performMRI on a pacemaker patient. The literature indicates a number ofprecautions that physicians should take in this case, including limitingthe power of the MRI RF Pulsed field (Specific Absorption Rate—SARlevel), programming the pacemaker to fixed or asynchronous pacing mode,and then careful reprogramming and evaluation of the pacemaker andpatient after the procedure is complete. There have been reports oflatent problems with cardiac pacemakers or other AIMDs after an MRIprocedure sometimes occurring many days later. Moreover, there are anumber of recent papers that indicate that the SAR level is not entirelypredictive of the heating that would be found in implanted leadwires ordevices. For example, for magnetic resonance imaging devices operatingat the same magnetic field strength and also at the same SAR level,considerable variations have been found relative to heating of implantedleadwires. It is speculated that SAR level alone is not a good predictorof whether or not an implanted device or its associated leadwire systemwill overheat.

There are three types of electromagnetic fields used in an MRI unit. Thefirst type is the main static magnetic field designated B₀ which is usedto align protons in body tissue. The field strength varies from 0.5 to3.0 Tesla in most of the currently available MRI units in clinical use.Some of the newer MRI system fields can go as high as 4 to 5 Tesla. Atthe recent International Society for Magnetic Resonance in Medicine(ISMRM), which was held on 5 and 6 Nov. 2005, it was reported thatcertain research systems are going up as high as 11.7 Tesla and will beready sometime in 2009. This is over 100,000 times the magnetic fieldstrength of the earth. A static magnetic field can induce powerfulmechanical forces and torque on any magnetic materials implanted withinthe patient. This would include certain components within the cardiacpacemaker itself and or leadwire systems. It is not likely (other thansudden system shut down) that the static MRI magnetic field can inducecurrents into the pacemaker leadwire system and hence into the pacemakeritself. It is a basic principle of physics that a magnetic field musteither be time-varying as it cuts across the conductor, or the conductoritself must move within the magnetic field for currents to be induced.

The second type of field produced by magnetic resonance imaging is thepulsed RF field which is generated by the body coil or head coil. Thisis used to change the energy state of the protons and elicit MRI signalsfrom tissue. The RF field is homogeneous in the central region and hastwo main components: (1) the magnetic field is circularly polarized inthe actual plane; and (2) the electric field is related to the magneticfield by Maxwell's equations. In general, the RF field is switched onand off during measurements and usually has a frequency of 21 MHz to 64MHz to 128 MHz depending upon the static magnetic field strength. Thefrequency of the RF pulse varies by the Lamor equation with the fieldstrength of the main static field where: RF PULSED FREQUENCY inMHz=(42.56) (STATIC FIELD STRENGTH IN TESLA).

The third type of electromagnetic field is the time-varying magneticgradient fields designated B_(x), B_(y), B_(z), which are used forspatial localization. These change their strength along differentorientations and operating frequencies on the order of 1 kHz. Thevectors of the magnetic field gradients in the X, Y and Z directions areproduced by three sets of orthogonally positioned coils and are switchedon only during the measurements. In some cases, the gradient field hasbeen shown to elevate natural heart rhythms (heart beat). This is notcompletely understood, but it is a repeatable phenomenon. The gradientfield is not considered by many researchers to create any other adverseeffects.

It is instructive to note how voltages and electro-magnetic interference(EMI) are induced into an implanted leadwire system. At very lowfrequency (VLF), voltages are induced at the input to the cardiacpacemaker as currents circulate throughout the patient's body and createvoltage drops. Because of the vector displacement between the pacemakerhousing and, for example, the tip electrode, voltage drop across theresistance of body tissues may be sensed due to Ohms Law and thecirculating current of the RF signal. At higher frequencies, theimplanted leadwire systems actually act as antennas where voltages(EMFs) are induced along their length. These antennas are not veryefficient due to the damping effects of body tissue; however, this canoften be offset by extremely high power fields (such as MRI pulsedfields) and/or body resonances. At very high frequencies (such ascellular telephone frequencies), EMI signals are induced only into thefirst area of the leadwire system (for example, at the header block of acardiac pacemaker). This has to do with the wavelength of the signalsinvolved and where they couple efficiently into the system.

Magnetic field coupling into an implanted leadwire system is based onloop areas. For example, in a cardiac pacemaker unipolar lead, there isa loop formed by the leadwire as it comes from the cardiac pacemakerhousing to its distal tip, for example, located in the right ventricle.The return path is through body fluid and tissue generally straight fromthe tip electrode in the right ventricle back up to the pacemaker caseor housing. This forms an enclosed area which can be measured frompatient X-rays in square centimeters. The average loop area is 200 to225 square centimeters. This is an average and is subject to greatstatistical variation. For example, in a large adult patient with anabdominal implant, the implanted loop area is much larger (greater than400 square centimeters).

Relating now to the specific case of MRI, the magnetic gradient fieldswould be induced through enclosed loop areas. However, the pulsed RFfields, which are generated by the body coil, would be primarily inducedinto the leadwire system by antenna action.

At the frequencies of interest in MRI, RF energy can be absorbed andconverted to heat. The power deposited by RF pulses during MRI iscomplex and is dependent upon the power (Specific Absorption Rate (SAR)Level) and duration of the RF pulse, the transmitted frequency, thenumber of RF pulses applied per unit time, and the type of configurationof the RF transmitter coil used. The amount of heating also depends uponthe volume of tissue imaged, the electrical resistivity of tissue andthe configuration of the anatomical region imaged. There are also anumber of other variables that depend on the placement in the human bodyof the AIMD and its associated leadwire(s). For example, it will make adifference how much EMF is induced into a pacemaker leadwire system asto whether it is a left or right pectoral implant. In addition, therouting of the lead and the lead length are also very critical as to theamount of induced current and heating that would occur. Also, distal tipdesign is very important as the distal tip itself can act as its ownantenna wherein eddy currents can create heating. The cause of heatingin an MRI environment is twofold: (a) RF field coupling to the lead canoccur which induces significant local heating; and (b) currents inducedbetween the distal tip and tissue during MRI RF pulse transmissionsequences can cause local Ohms Law (resistive) heating in tissue next tothe distal tip electrode of the implanted lead. The RF field of an MRIscanner can produce enough energy to induce leadwire RF voltages andresulting currents sufficient to destroy some of the adjacent myocardialtissue. Tissue ablation has also been observed. The effects of thisheating are not readily detectable by monitoring during the MRI.Indications that heating has occurred would include an increase inpacing threshold, venous ablation, Larynx or esophageal ablation,myocardial perforation and lead penetration, or even arrhythmias causedby scar tissue. Such long term heating effects of MRI have not been wellstudied yet for all types of AIMD leadwire geometries. There can also belocalized heating problems associated with various types of electrodesin addition to tip electrodes. This includes ring electrodes or padelectrodes. Ring electrodes are commonly used with a wide variety ofimplanted devices including cardiac pacemakers, and neurostimulators,and the like. Pad electrodes are very common in neurostimulatorapplications. For example, spinal cord stimulators or deep brainstimulators can include a plurality of pad electrodes to make contactwith nerve tissue. A good example of this also occurs in a cochlearimplant. In a typical cochlear implant there would be sixteen ringelectrodes placed up into the cochlea. Several of these ring electrodesmake contact with auditory nerves. Although there are a number ofstudies that have shown that MRI patients with active implantablemedical devices, such as cardiac pacemakers, can be at risk forpotential hazardous effects, there are a number of reports in theliterature that MRI can be safe for imaging of pacemaker patients when anumber of precautions are taken (only when an MRI is thought to be anabsolute diagnostic necessity). While these anecdotal reports are ofinterest, they are certainly not scientifically convincing that all MRIcan be safe. For example, just variations in the pacemaker leadwirelength can significantly affect how much heat is generated. A paperentitled, HEATING AROUND INTRAVASCULAR GUIDEWIRES BY RESONATING RF WAVESby Konings, et al., Journal of Magnetic Resonance Imaging, Issue12:79-85 (2000), does an excellent job of explaining how the RF fieldsfrom MRI scanners can couple into implanted leadwires. The paperincludes both a theoretical approach and actual temperaturemeasurements. In a worst-case, they measured temperature rises of up to74 degrees C. after 30 seconds of scanning exposure. The contents ofthis paper are incorporated herein by reference.

The effect of an MRI system on the function of pacemakers, ICDs,neurostimulators and the like, depends on various factors, including thestrength of the static magnetic field, the pulse sequence, the strengthof RF field, the anatomic region being imaged, and many other factors.Further complicating this is the fact that each patient's condition andphysiology is different and each manufacturer's pacemaker and ICDdesigns also are designed and behave differently. Most experts stillconclude that MRI for the pacemaker patient should not be consideredsafe.

It is well known that many of the undesirable effects in an implantedleadwire system from MRI and other medical diagnostic procedures arerelated to undesirable induced EMFs in the leadwire system and/or RFcurrents in its distal tip (or ring) electrodes. This can lead tooverheating of body tissue at or adjacent to the distal tip.

Distal tip electrodes can be unipolar, bipolar and the like. It is veryimportant that excessive current not flow at the interface between thedistal tip electrode and body tissue. In a typical cardiac pacemaker,for example, the distal tip electrode can be passive or of a screw-inhelix type as will be more fully described. In any event, it is veryimportant that excessive RF current not flow at this junction betweenthe distal tip electrode and for example, myocardial or nerve tissue.This is because tissue damage in this area can raise the capturethreshold or completely cause loss of capture. For pacemaker dependentpatients, this would mean that the pacemaker would no longer be able topace the heart. This would, of course, be life threatening for apacemaker dependent patient. For neurostimulator patients, such as deepbrain stimulator patients, the ability to have an MRI is equallyimportant.

Accordingly, there is a need for novel RF impeding and/or divertingcircuits, which are frequency selective and are constructed of passivecomponents for implantable leadwires. The purpose of these circuits isto prevent MRI induced energy from reaching the distal tip electrode orits interface with body tissue. By redirecting said energy to locationsat a point distant from the distal electrodes, this minimizes oreliminates hazards associated with overheating of said distal electrodesduring diagnostic procedures, such as MRI. Such circuits should decoupleand transfer energy which is induced from the MRI pulsed RF field to anenergy dissipating surface. The present invention fulfills these needsand provides other related advantages.

SUMMARY OF THE INVENTION

The present invention includes frequency selective impeding anddiverting (decoupling) circuits which transfer energy which is inducedfrom the MRI pulsed RF field to an energy dissipating surface (EDS). Inthis way, RF energy can be shunted harmlessly into the bulk of a probeor catheter, body tissues distant from the distal electrodes, or intoflowing blood or other body fluids thereby directing such energy awayfrom a distal tip electrode.

In other words, a novel energy dissipating surface is provided withmeans for decoupling RF signals from implantable leadwires selectivelyto said energy dissipating surface. Referring to Provisional ApplicationSer. No. 60/283,725, Paragraph 4.5, it is stated, “In the previousstudies, concerns have been raised about the safety of using metallicstructures in MR scanners. Radio frequency energy (MHz)—transmitted fromthe scanner in order to generate the MR signal—can be deposited on theinterventional device. This results in high electrical fields around theinstrument and local tissue heating. This heating tends to be mostconcentrated at the ends of the electrical structure.” This is certainlytrue of the implanted leadwires associated with AIMDs. “We can addressthis safety issue using the methods of the invention. The concern isthat the surface ring electrodes, which directly contact the tissue,could cause local tissue changes including burns.” The present inventionis extended beyond the leadwires of probes and catheters to include thedistal tip electrodes associated with the implanted leads of devicessuch as pacemakers, cardioverter defibrillators, neurostimulators andthe like. All of these devices have a distal electrode which contactsbody tissue in order to deliver pacing pulses or sense biologicactivity. It is extremely important that that interface junction notoverheat and cause localized tissue damage or burning.

The '725 Provisional Application explains the need to cut/remove theelectrodes from the circuit in the MHz frequency range. This isaccomplished with the inductor circuit elements. In the MHz frequencyrange, the surface ring electrodes are not connected to the rest of theelectrical leads. Therefore, the ends of the leads are now buried insideof the catheter. The coupled high electric fields will now be locatedinside of the catheter instead of in the tissue. This results insignificant reduction and unwanted tissue heating.

In the '725 Provisional Application, the inside of the catheter, ofcourse, includes a body with a specific thermal mass and specificthermal properties. Over time, it will rise in temperature and thereforeheat surrounding body tissue. However, this temperative rise is minimaldue to the large area and thermal mass of the catheter which acts as anenergy dissipating area or surface. Also, any such minimal heating thatdoes occur is in body tissue in an area that is distant from the therapyelectrode(s). Therefore, the ability for the pacing or stimuluselectrode to delivery energy in the proper location will not becompromised. By spreading the RF energy over a larger surface area (i.e.inside the catheter) the temperature rise is therefore reduced and theresulting small amount of heat is generally dissipated into bulk bodytissues instead of at a specific point.

This is accomplished through broad band filtering such as capacitancecoupling, or by resonant filtering such as creating resonant circuitsconsisting of a series inductor and capacitor. These general conceptsare described in U.S. Provisional Patent Application No. 60/283,725, andU.S. Patent Application Publication No. 2003/0050557, the contents ofwhich are incorporated herein by reference. Using series bandstopfilters is described in U.S. Pat. No. 7,363,090 and U.S. PatentApplication Publication Nos. 2007-0112398 A1, 2008-0071313 A1,2008-0049376 A1, 2008-0132987 A1, 2008-0116997 A1, the contents of whichare incorporated herein by reference.

There are three types of electromagnetic fields used in an MRI unit. Thefirst type is the main static magnetic field designated B₀ which is usedto align protons in body tissue. The field strength varies from 0.5 to 3Tesla in most of the currently available MRI units in present clinicaluse. The second electromagnetic field is the pulsed RF field which isgiven by the Lamor Frequency. The Lamor Frequency formula is 42.56(static field strength in Tesla)=RF frequency. For example, for a 1.5Tesla scanner, the frequency of the pulsed RF field is approximately 64MHz. The third type of field is the gradient field which is used tocontrol where the slice is that generates the image is located withinbody tissue.

The present invention is primarily directed to the pulsed RF fieldalthough it also has applicability to the gradient field as well.Because of the presence of the powerful static field, non-ferromagneticcomponents are presently used throughout the present invention. The useof ferromagnetic components is contraindicative because they have atendency to saturate or change properties in the presence of the mainstatic field.

In a broad sense, the present invention relates to a passive componentnetwork for an implantable leadwire of an active implantable medicaldevice (AIMD), comprising: (1) at least one leadwire having a lengthextending between and to a proximal end and a tissue-stimulating orbiological-sensing electrode at a distal tip end; and (2) a frequencyselective energy diversion circuit for diverting and/or impedinghigh-frequency energy away from the electrode to a point or an areaspaced from the electrode for dissipation of said high-frequency energy.The high-frequency energy may comprise an MRI frequency or a range ofMRI frequencies in megahertz selected from the group of frequenciescomprising 42.56 times strength in Teslas of an MRI scanner. Thefrequency selective energy diversion circuit may comprise a low passfilter such as a capacitor, an inductor, a Pi filter, a T filter, an LLfilter, or an “n” element filter. Moreover, the frequency selectiveenergy diversion circuit may comprise one or more series resonant LCtrap filters.

An energy dissipating surface is preferably disposed at a point or anarea spaced from the electrode, for example, within the blood flow of apatient. The energy dissipating surface may comprise a conductivehousing, a ring electrode, a sheath, an insulative body, or a thermallyconductive element. Moreover, the energy dissipating surface maycomprise convolutions or fins for increasing the surface area thereof,or may comprise a roughened surface. The roughened surface may be formedthrough plasma or chemical etching processes, porous or fractal coatingsor surfaces, whiskers, morphologically designed columnar structures,vapor, electron beam or sputter deposition of a high surface area energyconductive material, or by application of carbon nanotubes.

A leadwire housing may be provided which supports the leadwire distaltip electrode at one end thereof. The leadwire housing includes aconductive housing portion forming an energy dissipating surface, and aninsulator housing portion between the leadwire distal tip electrode andthe conductive housing portion.

An impeding circuit may be associated with the diversion circuit forraising the high-frequency impedance of the leadwire. The impedancecircuit is disposed between the diversion circuit and the distal tip endof the at least one leadwire. The impeding circuit may comprise aninductor or a bandstop filter.

The leadwire may comprise a portion of a probe or a catheter, or it maycomprise a pair of leadwires each having a length extending between andto a proximal end and a tissue-stimulating or biological-sensingelectrode at a distal tip end. The diversion circuit may couple each ofthe leadwires to an energy dissipating surface disposed at a point or anarea spaced from each of said electrodes. The diversion circuit may alsobe coupled between the pair of leadwires.

The diversion circuit may be mounted within a conductive housing whichprotects the diversion circuit from direct contact with patient bodyfluids. In this case, the conductive housing is preferably hermeticallysealed. The active implantable medical device (AIMD) may comprise a deepbrain stimulator. In this case, the conductive housing would be adaptedfor mounting in thermal communication with a patient's skull and/ordura. An electrode shaft assembly having a proximal end is carried bythe conductive housing, and the leadwire(s) extends through theelectrode shaft assembly and has the distal tip end electrode inposition for contacting patient brain tissue.

Further, the energy dissipating surface may be disposed within aninsulative sheath, and comprise at least a portion of a handle or pistolgrip for a steerable probe or catheter. The energy dissipating surfacemay also comprise a plurality of spaced-part energy dissipatingsurfaces.

In other embodiments, a tether may be disposed between and conductivelycoupled with the electrode and the energy dissipating surface. Theelectrode may comprise a paddle electrode disposed on one side of apaddle, wherein the energy dissipating surface is disposed on a secondside of the paddle. In this case, the frequency selective diversioncircuit would comprise a capacitive element disposed within the paddlebetween the electrode and the energy dissipating surface.

Other features and advantages of the present invention will becomeapparent from the following more detailed description, taken inconjunction with the accompanying drawings which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is wire-formed diagram of a generic human body showing a numberof active implantable medical devices (AIMDs);

FIG. 2 is a diagrammatic view of a typical probe or catheter;

FIG. 3 is a diagrammatic view of the interior of the prober or catheterof FIG. 2;

FIG. 4 is an electrical circuit diagram of the structure shown in FIG.3, with a general impedance element connected between leadwires;

FIG. 5 is an electrical diagrammatic view similar to FIG. 4,illustrating a capacitor representing a frequency dependent reactiveelement between the leadwires;

FIG. 6 is a view similar to FIG. 5, wherein the general reactanceelement has been replaced by a capacitor in series with an inductor;

FIG. 7 is a view similar to FIGS. 4-6, showing the addition of seriesfrequency selective reactances;

FIG. 8 is similar to FIG. 3, showing a low frequency model of thecatheter and associated leads described in FIG. 2;

FIG. 9 is a view similar to FIGS. 3-8, illustrating how the distal ringsare electrically isolated at a high frequency;

FIG. 10 is a view similar to FIGS. 3-9, showing the addition of seriesinductor components added to the frequency selective elements 20;

FIG. 11 is similar to FIGS. 3-10, illustrating frequency selectiveelements which incorporate parallel resonant inductor and capacitorbandstop filters;

FIG. 12 is a perspective and somewhat schematic view of a prior artactive implantable medical device (AIMD) including a leadwire directedto the heart of a patient;

FIG. 13 is a diagram of a unipolar active implantable medical device;

FIG. 14 is a diagram similar to FIG. 13, illustrating a bipolar AIMDsystem;

FIG. 15 is a diagram similar to FIGS. 13 and 14, illustrating a bipolarleadwire system with a distal tip and ring electrodes, typically used ina cardiac pacemaker;

FIG. 16 is a tracing of an exemplary patient X-ray showing an implantedpacemaker and cardioverter defibrillator and corresponding leadwiresystem;

FIG. 17 is a line drawing of an exemplary patient cardiac X-ray of abi-ventricular leadwire system;

FIG. 18 illustrates a bipolar cardiac pacemaker leadwire showing thedistal tip and the distal ring electrodes;

FIG. 19 is an enlarged schematic illustration of the area indicated byLine 19-19 in FIG. 18, showing details of the bipolar pacemaker leadwiresystem;

FIG. 19A is similar to FIG. 19, but depicts an active fixation tip for abipolar pacemaker leadwire system;

FIG. 20 is similar to FIG. 19, except that the twisted or coaxialelectrode wires have been straightened out;

FIG. 21 is similar to FIG. 20 and incorporates electrical featuresdiscussed in FIGS. 2-11;

FIG. 22 is similar to a portion of FIGS. 20 and 21, and depicts an L-Ctrap filter coupled between a distal tip electrode wire and acylindrical ring electrode;

FIG. 23 is a schematic low frequency model illustration operation of theembodiment depicted generally at FIGS. 7-8;

FIG. 24 is a schematic diagram similar to FIG. 23, but shows a highfrequency model of the embodiment depicted generally at FIGS. 7 and 9;

FIG. 25 is a line drawing of a human heart with cardiac pacemaker dualchamber bipolar leads shown in the right ventricle and the right atrium;

FIG. 26 is a schematic diagram illustration an energy dissipatingsurface in spaced relation with tip and ring electrodes;

FIG. 27 a schematic diagram depicting a typical quad polarneurostimulation lead system;

FIG. 28 is a somewhat schematic side view of the human head with a deepbrain stimulation electrode shaft assembly implanted therein;

FIG. 29 is an enlarged sectional view corresponding generally with theencircled region 29-29 of FIG. 28;

FIG. 29A is a further enlarged and somewhat schematic view correspondinggenerally with the encircled region 29A-29A of FIG. 29;

FIG. 29B is an enlarged and somewhat schematic view correspondinggenerally with the encircled region 29B-29B of FIG. 29;

FIG. 30 is a sectional view of an hermetically sealed electrode assemblydesigned for contact with body fluid;

FIG. 31 is a perspective sectional view of a housing portion of thesealed electrode assembly of FIG. 30;

FIG. 31A is an enlarged sectional view corresponding generally with theencircled region 31A-31A of FIG. 31, and illustrating the principle ofincreasing the surface area of the energy dissipating surface;

FIG. 32 is a schematic circuit diagram corresponding with the sealedelectrode assembly of FIG. 30;

FIG. 33A is a perspective view of an exemplary monolithic capacitor foruse in the circuit of FIG. 32;

FIG. 33B is a perspective view of an exemplary unipolar feedthroughcapacitor for use in the circuit of FIG. 32;

FIG. 34 is a sectional view similar to FIG. 30 and depicts analternative embodiment wherein an inductor element is wound or printedabout a central mandrel;

FIG. 35 is a sectional view similar to FIGS. 30 and 34, but illustratesa further alternative embodiment of the invention with alternative meansfor decoupling signals from a leadwire to an energy dissipating surface;

FIG. 36 is a schematic circuit diagram corresponding with the sealedelectrode assembly of FIG. 35;

FIG. 37 is an attenuation versus frequency chart for various types oflow pass filters;

FIG. 38 shows schematic circuit diagrams for different types of low passfilters charted in FIG. 37;

FIG. 39 is a schematic circuit diagram illustrating an L-C trap filter;

FIG. 39A depicts a resonant frequency equation for the L-C trap filterof FIG. 39;

FIG. 40 is an impedance versus frequency chart for the L-C trap filterof FIG. 39;

FIG. 41 is a sectional view similar to FIGS. 30, 34 and 35, but showsstill another alternative embodiment of the invention for decoupling RFsignals from an electrode leadwire;

FIG. 42 is a schematic circuit diagram corresponding with the sealedelectrode assembly of FIG. 41;

FIG. 43A illustrates a typical chip inductor for use in the sealedelectrode assembly of FIG. 41;

FIG. 43B illustrates a typical chip capacitor for use in the sealedelectrode assembly of FIG. 41;

FIG. 44 is an impedance versus frequency chart for the dual L-C trapfilter embodiment of FIG. 41;

FIG. 45 is a schematic representation of an implantable medical devicebipolar leadwire system;

FIG. 46 is an enlarged and somewhat schematic sectional view takengenerally on the line 46-46 of FIG. 45;

FIG. 47 is an isometric view of a bipolar feedthrough capacitor for usein the device of FIGS. 45-56;

FIG. 48 is a schematic circuit diagram corresponding with the embodimentshown in FIGS. 45-46;

FIG. 49 is a schematic circuit diagram illustrating a bipolar leadassembly with distal tip and ring electrodes shown at a suitabledistance from an energy dissipation surface (EDS);

FIG. 50 is a schematic circuit diagram similar to FIG. 49, except that apair of capacitors are used;

FIG. 51 is a schematic circuit diagram illustrating a band stop filtermodified to include a pair of diodes in a parallel or back-to-backconfiguration;

FIG. 52 is a schematic circuit diagram similar to FIG. 50, except thattransient voltage suppressors are installed in parallel relation witheach of the bandstop filter elements;

FIG. 53 is a schematic circuit diagram depicting a general filterelement constructed in accordance with any one of the embodiments shownand described herein, wherein the filter element is coupled between thedistal and proximal ends of a leadwire or the like, for dissipating RFenergy or heat to an adjacent energy dissipating surface;

FIG. 54 is a schematic circuit diagram similar to FIG. 53, but showingalternative design considerations;

FIG. 55 depicts in somewhat schematic form a probe or catheterconstructed in accordance with the present invention;

FIG. 56 is an illustration similar to FIG. 55, illustrating analternative embodiment wherein the energy dissipating surface has beenconvoluted so that its surface area has been increased;

FIG. 57 is similar to FIG. 56, except that instead of convolutions, finshave been added to the energy dissipating surface;

FIG. 58 is similar to FIGS. 56 and 57, except that the energydissipating surface has its surface area increased through varioussurface roughening processes;

FIG. 59 is an enlarged, fragmented sectional view taken along the line59-59 from FIG. 58, illustrating a roughened surface formed through, forexample, plasma or chemical etching, or the like;

FIG. 60 is a view similar to FIG. 59, and illustrates the use of carbonnanotubes or fractal coatings to increase the surface area of the energydissipating surface;

FIG. 61 is an illustration of a steerable catheter;

FIG. 62 is an enlarged section view taken generally along the line 62-62from FIG. 61;

FIG. 63 is a schematic view of a probe or catheter similar to FIG. 55,except that the number of individual energy dissipating surfaces havebeen provided in distinct and spaced-apart segments;

FIG. 64 is a fragmented top plan view of an exemplary paddle electrodeembodying the present invention;

FIG. 65 is a bottom plan view of the paddle electrode shown in FIG. 64;

FIG. 66 is an enlarged sectional view taken generally along the line66-66 in FIG. 64;

FIG. 67 is a top plan view of a different type of paddle lead structurein comparison with that shown in FIGS. 64-66;

FIG. 68 is an enlarged electrical schematic view taken generally of thearea indicated by the line 68-68 in FIG. 67;

FIG. 69 is a schematic illustration similar to FIG. 28, showing use of atethered energy dissipating surface in accordance with the presentinvention;

FIG. 70 is an enlarged sectional view of the area indicated by the line70-70 in FIG. 69; and

FIG. 71 is an enlarged, somewhat schematic illustration of thecomponents found within the area designated by the line 71-71 in FIG.70.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates various types of active implantable medical devicesreferred to generally by the reference numeral 10 that are currently inuse. FIG. 1 is a wire formed diagram of a generic human body showing anumber of exemplary implanted medical devices. 10A is a family ofimplantable hearing devices which can include the group of cochlearimplants, piezoelectric sound bridge transducers and the like. 10Bincludes an entire variety of neurostimulators and brain stimulators.Neurostimulators are used to stimulate the Vagus nerve, for example, totreat epilepsy, obesity and depression. Brain stimulators are similar toa pacemaker-like device and include electrodes implanted deep into thebrain for sensing the onset of the seizure and also providing electricalstimulation to brain tissue to prevent the seizure from actuallyhappening. 10C shows a cardiac pacemaker which is well-known in the art.10D includes the family of left ventricular assist devices (LVAD's), andartificial hearts, including the recently introduced artificial heartknown as the Abiocor. 10E includes an entire family of drug pumps whichcan be used for dispensing of insulin, chemotherapy drugs, painmedications and the like. Insulin pumps are evolving from passivedevices to ones that have sensors and closed loop systems. That is, realtime monitoring of blood sugar levels will occur. These devices tend tobe more sensitive to EMI than passive pumps that have no sense circuitryor externally implanted leadwires. 10F includes a variety of implantablebone growth stimulators for rapid healing of fractures. 10G includesurinary incontinence devices. 10H includes the family of pain reliefspinal cord stimulators and anti-tremor stimulators. 10H also includesan entire family of other types of neurostimulators used to block pain.10I includes a family of implantable cardioverter defibrillator (ICD)devices and also includes the family of congestive heart failure devices(CHF). This is also known in the art as cardio resynchronization therapydevices, otherwise known as CRT devices. 10J illustrates an externallyworn pack. This pack could be an external insulin pump, an external drugpump, an external neurostimulator or even a ventricular assist device.10K illustrates the insertion of an external probe or catheter. Theseprobes can be inserted into the femoral artery, for example, or in anyother number of locations in the human body.

Referring to U.S. Publication No. U.S. 2003/0050557, Paragraphs 79through 82, the contents of which are incorporated herein, metallicstructures, particularly leadwires, are described that when placed inMRI scanners, can pick up high electrical fields which results in localtissue heating. This heating tends to be most concentrated at the endsof the electrical structure (either at the proximal or distal leadends). This safety issue can be addressed using the disclosed systemsand methods of the present invention. The concern is that the distalelectrodes, which directly contact body tissue, can cause local tissueburns.

Reference is made to U.S. Publication No. 2003/0050557 drawing, FIGS. 1Athrough 1G. These figures have been redrawn herein as FIGS. 2 through 11and are described as follows in light of the present invention.

FIG. 2 is a diagrammatic view of a typical prior art device 10 such as aprobe or catheter. There are two leadwires 12 and 14 which threadthrough the center of the illustrative probe or catheter and terminaterespectively in a corresponding pair of distal conductive electroderings 16 and 18. Leadwires 12 and 14 are electrically insulated fromeach other and also electrically insulated from any metallic structureslocated within the catheter body. The overall catheter body is generallyflexible and is made of biocompatible materials, which also havespecific thermal properties. In addition to flexibility, probes andcatheters are typically steerable. It is well known that a push-pullwire (not shown in FIG. 2) can be run down the center of the catheter orprobe in a lumen and then be attached to a catheter handle or pistolgrip or other device so that the physician can carefully steer or threadthe probe or catheter through the torturous path of the venous system,even into the ventricles of the heart. Such probes and catheters, forexample, can be used for electrical mapping inside of a heart chamber,or for application of RF energy for ablation, which is used to treatcertain cardiac arrhythmias. Probes and catheters have wide applicationto a variety of other medical applications. There are also combinedcatheters that can do electrical mapping and can also perform RFablation. When the physician finds the area of arrhythmic electricalactivity and wishes to ablate, he activates a switch which applies RFenergy to the tip of the catheter (see, e.g., FIG. 55, which will bediscussed herein in more detail). This would involve a third electroderight at the catheter tip of FIG. 2 (not shown). It would be extremelyvaluable if the catheter could be guided during real-time MRI imaging.This is important because of MRI's incredible ability to image softtissue. In addition, when one is doing deliberate ablation, for example,around a pulmonary vein, it is important that a full circle of scartissue be formed, for example, to stop atrial fibrillation. MRI has theability to image the scar as it is being formed (for example, see U.S.Pat. No. 7,155,271). However, it would be highly undesirable if the MRIRF energy that is coupled to the leadwires caused the distal ablationtip or the electrode rings to overheat at an improper time, which couldburn or ablate healthy tissues.

FIG. 3 shows the interior taken from FIG. 2 showing leadwires 12 and 14which are routed to the two distal electrodes 16 and 18 as previouslydescribed in FIG. 2.

FIG. 4 shows the electrical circuit of FIG. 3 with a general frequencyselective impedance element 20 connected between leadwires 12 and 14. Inthe present invention, the impedance element 20 can consist of a numberof frequency selective elements as will be further described. Ingeneral, the first conductive leadwire 12 is electrically coupled to thefirst electrode 116, the second conductive leadwire 14 is electricallycoupled to the second electrode 18, and the frequency dependent reactiveelement 20 electrically couples the first and second leadwires 12 and 14such that high frequency energy is conducted between the first leadwire12 and the second leadwire 14.

Referring once again to FIG. 4, the frequency dependent reactive element20 tends to be electrically invisible (i.e., a very high impedance) atselected frequencies. The reactive element is desirably selective suchthat it would not attenuate, for example, low frequency biologicalsignals or RF ablation pulses. However, for high frequency MRI RF pulsedfrequencies (such as 64 MHz), this frequency reactive element 20 wouldlook more like a short circuit. This would have the effect of sendingthe energy induced into the leadwires 12 and 14 by the MRI RF field backinto the catheter body itself into which the leadwires are embedded. Inother words, there are both RF energy and thermal conductivity to theprobe or catheter body or sheath or shield which becomes an energydissipating surface all along the lengths of leadwires 12 and 14 suchthat MRI induced energy that is present in these leadwires is divertedand converted to heat into the interior and along the catheter bodyitself. This prevents the heat build up at the extremely sensitivelocations right at the ring electrodes 16 and 18 which are in intimateand direct contact with body tissue. In addition, the amount oftemperature rise is very small (just a few degrees) because of theenergy being dissipated over such a relatively high surface area. Aspreviously mentioned, the high frequency RF pulsed energy from an MRIsystem can couple to implanted leads. This creates electromagneticforces which can result in current flowing through the interface betweenelectrodes that are in contact with body tissue. If this current reachescertain values, body tissue could be damaged by heat build-up. This cancreate scar tissue formation, tissue damage or even tissue necrosis suchto the point where the AIMD can no longer deliver appropriate therapy.In certain situations, this can be life threatening for the patient.

FIG. 5 shows a capacitor 22 which represents one form of the frequencydependent reactive element 20 previously described in FIG. 4. In thiscase, the reactive element comprises a simple capacitor 22 connectedbetween the first conductor or leadwire 12 and the second conductor orleadwire 18 and will have a variable impedance vs. frequency. Thefollowing formula is well known in the art: X_(c)=1/(2πfc). Referring tothe equation, one can see that since frequency (f) is in thedenominator, as the frequency increases, the capacitive reactance inohms decreases. With a large number in the denominator, such as the RFpulsed frequency of a 1.5 Tesla MRI system, which is 64 MHz, thecapacitive reactance drops to a very low number (essentially a shortcircuit). By shorting the leadwires together at this one frequency, thisdiverts and prevents the RF energy from reaching the distal ringelectrodes 16 and 18 and being undesirably dissipated as heat into bodytissue. Referring once again to FIG. 4, one can see that the impedanceelement 20 thereby diverts the high frequency RF energy back into theleadwires 12 and 14. By spreading this energy along the length ofleadwires 12 and 14, it is converted to heat, which is dissipated intothe main body of the probe, catheter or insulation sheath. In this way,the relatively large thermal mass of the probe or catheter becomes anenergy dissipating surface and any temperature rise is just a fewdegrees C. In general, a few degrees of temperature rise is not harmfulto body tissue. In order to cause permanent damage to body tissue, suchas an ablation scar, it generally requires temperatures of approximately50° C. In summary, the frequency dependent reactive element 20, whichmay comprise a capacitor 22 as shown in FIG. 5, forms a diversioncircuit such that high frequency energy is diverted away from the distalelectrodes 16 and 18 along the leadwires 12 and 14 to a point that isdistant from the electrodes 16 and 18, at which point the energy isconverted to heat.

FIG. 6 describes an even more efficient way of diverting high frequencyenergy away from the electrode and accomplishing the same objective. Thegeneral reactance element 20 described in FIG. 4 is shown in FIG. 6 tocomprise the capacitor 22 in series with an inductor 24 to form an L-Ctrap circuit. There is a particular frequency (f_(r)) at which thecapacitive reactance X_(C) and the inductive reactance X_(L) are equaland opposite and tend to cancel each other out. If there are no lossesin such a system, this results in a perfect short circuit betweenleadwires 12 and 14 at the resonant frequency. The frequency ofresonance is given by the equation

${f_{r} = \frac{1}{2\pi\sqrt{LC}}},$wherein f_(r) is the frequency of resonance in Hertz, L is theinductance in henries, and C is the capacitance in farads.

FIG. 7 illustrates any of the aforementioned frequency dependentimpedance elements 20 with the addition of series frequency selectivereactances 26 and 28. The addition of series impedance further impedesor blocks the flow of high frequency MRI induced currents to the ringelectrodes 16 and 18 as will be more fully described in the followingdrawings.

FIG. 8 is the low frequency model of FIG. 4, 5 or 6. In this regard,FIG. 8 is identical to FIG. 3, in that, once again it shows theelectrical leadwires 12 and 14 connected to the distal ring electrodes16 and 18 of the probe or catheter 10. In the low frequency model, thefrequency reactive impedance elements 20 disappear because at lowfrequency their impedances approach infinity. Of course, leads in aprobe or catheter are electrically equivalent to leads used for cardiacpacemakers, implantable cardioverter defibrillators, neurostimulatorsand the like. For example, reference is made to U.S. Pat. No. 7,363,090,the contents of which are incorporated herein. Accordingly, anydiscussion related to probes or catheters apply equally to leadwires forall active implantable medical devices as described in FIG. 1, and viceversa. Referring once again to FIG. 8, this is also the low frequencymodel of the circuits shown in FIG. 7. At low frequency, the frequencyselective or reactive component 20 tends to look like a very high orinfinite impedance. The series reactive or frequency variable elements26 and 28 at low frequency tend to look like a very low impedance orshort circuit. Accordingly, they all tend to disappear as shown in FIG.8.

FIG. 9 is a high frequency model that illustrates how the distalelectrodes or rings 16 and 18 are electrically isolated at highfrequency by shorting leadwires 12 and 14 at location 30. As previouslymentioned, such shorting or current diverting could be accomplished by aseries resonant L-C trap circuit. FIG. 9 also shows the electrodes 16and 18 as cut or disconnected and electrically isolated from the rest ofthe circuit. This is because, at very high frequency, series selectivefrequency (reactive) elements 26 and 28 tend to look like a very highimpedance or an open circuit. In summary, by reactive elements 20, 26and 28 acting cooperatively, reactive element 20 diverts the highfrequency energy while at the same time reactive elements 26 and 28impede the high frequency RF energy. Accordingly, in the ideal case, theequivalent circuit of FIG. 9 is achieved. The high frequency MRI RFenergy cannot reach the distal ring electrodes 16, 18 and causeundesirable heating at that critical tissue interface location.

FIG. 10 shows any of the previously described diverting frequencyselective elements 20 in combination with series reactance components26′ and 28′ shown in the form of a pair of inductors. It is well knownto electrical engineers that the inductive reactance in ohms is given bythe equation X_(L)=2πfL. In this case the frequency term (f) is in thenumerator. Accordingly, as the frequency increases, the reactance (ohms)of the inductors also increases. When the frequency is very high (suchas 64 MHz) then the reactance in ohms becomes extremely high (ideallyapproaches infinity and cuts off the electrodes). By having a shortcircuit or very low impedance between the leadwires 12 and 14 and then,at the same time, having a very high impedance in series with theelectrodes from inductors 26′ and 28′, this provides a very high degreeof attenuation to MRI RF pulsed frequencies thereby preventing suchenergy from reaching the distal ring electrodes 16 and 18. In FIG. 10,the line-to-line selective impedance element 20 diverts high frequencyenergy back into leadwires 12 and 14 while at the same time the seriesinductors 24′ and 25′ impede (or cut-off) high frequency energy. Whenthe line-to-line element 20 is a capacitor 22 as shown in FIG. 5, thenthis forms what is known in the prior art as an L section low passfilter, wherein the capacitor 22 electrically cooperates with theinductors 26′ and 28′ (FIG. 10) to form a 2-element low pass filter. Bydefinition, a low pass filter allows low frequencies such as biologicalsignals to pass to and from the distal electrodes through freely withoutattenuation while at the same time provides a high degree of attenuationto undesirable high frequency energy. It will be obvious to thoseskilled in the art that FIG. 5 describes a single element low passfilter, and that FIG. 10 describes a 2-element or L-section low passfilter. Moreover, any number of inductor and capacitor combinations canbe used for low pass filters, including 3-element Pi or T circuits, LL,5-element or even “n” element filters.

FIG. 11 offers an even greater performance improvement over thatpreviously described in FIG. 10. In FIG. 1, modified frequency selectiveelements 26″ and 28″ each incorporate a parallel resonant inductor andcapacitor which is also known in the industry as a band stop filter. TheL-C components for each of the reactive elements 26″ and 28″ arecarefully chosen such that each of the band stop filters 26″ and 28″ isresonant, for example, at the pulsed resonant frequency of an MRIscanner. The pulsed resonant frequency of an MR scanner is given by theLamor equation wherein the RF pulsed frequency in megahertz is equal to42.56 times the static field strength. For example, for a popular 1.5Tesla scanner, the RF pulsed frequency is 64 MHz. Common MR scannersthat are either in use or in development today along with their RFpulsed frequencies include: 0.5 Tesla-21 MHz; 1.5 Tesla-64 MHz; 3Tesla-128 MHz; 4 Tesla-170 MHz; 5 Tesla-213 MHz; 7 Tesla-300 MHz; 8Tesla-340 MHz; and 9.4 Tesla-400 MHz. When the band stop filters 26″ and28″ are resonant at any one of these RF pulsed frequencies, then theseelements tend to look like an open circuit which impedes the flow of RFcurrent to distal electrodes. When compatibility with different types ofMR scanners is required, for example, 1.5, 3 and 5 Tesla, then threeseparate band stop filter elements in parallel may comprise the reactiveelement 26 (FIG. 7), and three separate band stop filter elements inparallel may comprise the reactive element 28 (FIG. 7). Each of thesewould have their L and C components carefully selected so that theywould be resonant at different frequencies. For example, in the case ofMR scanners operating at 1.5, 3 and 5 Tesla, the three band stop filterscomprising the reactive element as well as the three band stop filterscomprising the reactive element 25 would be resonant respectively at 64MHz, at 128 MHz, and at 170 MHz. The resonant frequencies of the bandstop filter elements could be selected such that they are resonant atthe operating frequency of other emitters that the patient may encountersuch as diathermy and the like. The use of band stop filters is morethoroughly described in U.S. Pat. No. 7,363,090 and Patent PublicationNos. US 2007-0112398 A1, US 2007-0288058, US 2008-0071313 A1, US2008-0049376 A1, US 2008-0161886 A1, US 2008-0132987 A1 and US2008-0116997 A1, the contents of which are incorporated herein.

Referring now to FIG. 12, a prior art active implantable medical device(AIMD) 10 is illustrated. In general, the AIMD 10 could, for example, bea cardiac pacemaker (10C in FIG. 1) which is enclosed by a titaniumhousing 32 as indicated. The titanium housing 32 is hermetically sealed,however, there is a point where conductors such as the illustrativeconductors 34 a, 34 b, 34 c and 34 d must ingress and egress relative tothe housing 32. This is accomplished by providing a hermetic terminalassembly 36. Hermetic terminal assemblies are well known and generallyconsist of a ferrule 38 which is laser welded to the titanium housing 32of the AIMD 10. In FIG. 12, four conductors 34 a-34 d are shown forconnection to a corresponding number of leadwires, such as theillustrative leadwire 12 b shown for coupling to the conductor 34 b. Inthis configuration, the four leadwires coupled respectively to theconductors 34 a-34 d comprise a typical dual chamber bipolar cardiacpacemaker.

Connectors 40 commonly known as IS-1 connectors are designed to pluginto mating receptacles 42 on a header block 44 on the pacemaker housing32. These are low voltage (pacemaker) leadwire connectors covered by anInternational Standards Organization (ISO) standard IS-1. Higher voltagedevices, such as implantable cardioverter defibrillators, are covered bya standard known as the ISO DF-1. A new standard was recently publishedthat will integrate both high voltage and low voltage connectors into anew miniature quadpolar connector series known as the ISO IS-4 standard.Leads plugged into these connectors are typically routed in a pacemakeror ICD application down into the right ventricle and right atrium of theheart 46. There are also new generation devices that have beenintroduced to the market that couple leads to the outside of the leftventricle. These are known as biventricular devices and are veryeffective in cardiac resynchronization therapy (CRT) and treatingcongestive heart failure (CHF).

In FIG. 12, one can see, for example, the conductors 34 a and 34 b thatcould be coupled by leads routed, for example, to distal tip and ringelectrodes within the right ventricle of the heart 46. The other pair ofconductors 34 c and 34 d could be coupled by leads routed to distal tipand ring electrodes within the right atrium of the heart 46. There isalso an RF telemetry pin antenna 48 which is not connected to the IS-1or DS-1 connector block. This acts as a short stub antenna for pickingup telemetry signals that are transmitted from the outside of the device10.

It should be obvious to those skilled in the art that all of thedescriptions herein are equally applicable to other types of AIMDs.These include implantable cardioverter defibrillators (ICDs),neurostimulators, including deep brain stimulators, spinal cordstimulators, cochlear implants, incontinence stimulators and the like,and drug pumps. The present invention is also applicable to a widevariety of minimally invasive AIMDs. For example, in certain hospitalcath lab procedures, one can insert an AIMD for temporary use such as aprobe, catheter or femoral artery ICD. Ventricular assist devices alsocan fall into this type of category. This list is not meant to belimiting, but is only example of the applications of the noveltechnology currently described herein. In the following description,functionally equivalent elements shown in various embodiments will oftenbe referred to utilizing the same reference number.

FIG. 13 is a general diagram of a unipolar active implantable medicaldevice 10 and related system, wherein reference numbers common withthose used in FIG. 12 refer to common structural and/or functionalcomponents. The housing 32 of the active implantable medical device 10is typically titanium, ceramic, stainless steel or the like. Inside ofthe device housing are the AIMD electronic circuits. Usually AIMDsinclude a battery, but that is not always the case. A leadwire or lead12 is routed from the AIMD 10 to a point 50 typically including orcomprising an electrode embedded in or affixed to body tissue. In thecase of a spinal cord stimulator 10H (FIG. 1), the distal tip 50 couldbe in the spinal cord. In the case of a deep brain stimulator 10B (FIG.1), the distal electrode 50 would be placed deep into the brain, etc. Inthe case of a cardiac pacemaker 10C (FIG. 1), the distal electrode 50would typically be placed in the cardiac right ventricle.

FIG. 14 is very similar to FIG. 13 except that it depicts a bipolardevice 10 and related system. In this case, a first leadwire 12 iscoupled to a first distal electrode 50, and a second distal electrode 52and associated leadwire 14 provide an electric circuit return pathbetween the two distal electrodes 50 and 52. In the case of a cardiacpacemaker 10C as shown in FIG. 15, this would be known as a bipolarleadwire system with one of the electrodes known as the distal tipelectrode 50 and the other electrode which would float in the blood poolknown as the ring electrode 52 (see FIG. 15). In contrast, theelectrical return path in FIG. 13 is between the distal electrode 50through body tissue to the conductive housing 32 of the implantableunipolar medical device 10.

In all of these applications, the patient could be exposed to the fieldsof an MRI scanner or other powerful emitter used during a medicaldiagnostic procedure. Currents that are directly induced in the leadwiresystem, such as the illustrative leadwires 12 and 14, can cause heatingby P=I²R (Ohm's law) losses in the leadwire system or by heating causedby current flowing in body tissue. If these currents become excessive,the associated heating can cause damage or even destructive ablation tobody tissue.

The distal tip electrode 50 is designed to be implanted against or intoor affixed (screwed into) to the actual myocardial tissue of the heart46. The ring electrode 52 is designed to float in the blood pool.Because the blood is flowing and is thermally conductive, some peoplefeel that the ring structure 52 is substantially cooled. However, thisis only in theory. Studies have shown, upon lead removal that the entirearea of the tip and the ring can become overgrown and embedded in bodytissue and thereby thoroughly encapsulated. Accordingly, in somepacemaker patients, both the distal tip and ring can become thermallyinsulated by surrounding body tissue and can readily heat up due to theRF pulsed currents of an MRI field.

FIG. 16 is a tracing of an actual patient X-ray. This particular patientrequired both a cardiac pacemaker 10C and an implantable cardioverterdefibrillator (ICD) 101. The corresponding implantable leadwire system,as one can see, makes for a very complicated antenna and loop couplingsituation. The reader is referred to the article entitled, “Estimationof Effective Lead Loop Area for Implantable Pulse Generator andImplantable Cardioverter Defibrillators” provided by the AAMI PacemakerEMC Task Force.

In FIG. 16, one can see that from the pacemaker 10C, there is anelectrode 54 and 54′ in both the right atrium and in the rightventricle. Both these involve a separate tip and ring electrode (notshown in FIG. 16). In the industry, this is known as a dual chamberbipolar leadwire system. It will be obvious to those skilled in the artthat any of the passive frequency selective networks, as previouslydescribed in FIGS. 2 through 11, can be incorporated into the leadwiresas illustrated in the X-ray tracing of FIG. 16. One can also see thatthe implantable cardioverter defibrillator (ICD) 101 is associated withan electrode 56 implanted directly into the right ventricle. Itsshocking tip and perhaps its superior vena cava (SVC) shock coil wouldalso require the passive, frequency selective diverter and/or impedingfilters of FIGS. 2-11 of the present invention so that MRI exposurecannot induce excessive currents into the associated leadwires orelectrodes. Modern implantable cardioverter defibrillators (ICDs)incorporate both pacing and cardioverting (shock) features. Accordingly,it is becoming quite rare for a patient to have a leadwire layout asshown in the X-ray of FIG. 16. However, the number of electrodes remainsthe same. There are also newer combined pacemaker/ICD systems whichinclude biventricular pacemaking (pacing of the left ventricle). Thesesystems can have as many as 9 to even 12 leadwires.

FIG. 17 is a line drawing of an actual patient cardiac X-ray of one ofthe newer bi-ventricular leadwire systems with various types ofelectrode tips 54, 54′ and 56 shown. The new bi-ventricular systems arebeing used to treat congestive heart failure, and make it possible toimplant leads outside of the left ventricle. This makes for a veryefficient pacing system; however, the implantable leadwire system isquite complex. When a leadwire system, such as those described in FIGS.12, 13, 14, 15, 16 and 17, are exposed to a time varying electromagneticfield, electric currents can be induced into the electrodes of suchleadwire systems. For the bi-ventricular system, a passive componentfrequency selective network of FIGS. 2-11 would need to be placed inconjunction with each of the three distal tips and ring electrodes tocorresponding energy dissipating surfaces.

The word passive is very important in this context. Active electroniccircuits, which are defined as those that require power, do not operatevery well under very high intensity electromagnetic field conditions.Active electronic filters, which generally are made from microelectronic chips, have very low dynamic range. Extremely high fieldsinside an MRI chamber would tend to saturate such filters and make thembecome nonlinear and ineffective. Accordingly, frequency selectivenetworks are preferably realized using non-ferromagnetic passivecomponent elements. Passive component elements are capable of handlingvery high power levels without changing their characteristics orsaturating. Moreover, the inductor elements are preferably made frommaterials that are not ferromagnetic. The reason for this is that MRImachines have a very powerful main static magnetic field (B₀). Thispowerful static magnetic field tends to saturate ferrite elements andwould thereby change dramatically the value of the inductance component.Accordingly, virtually all inductor elements are fabricated without theuse of ferrites, nickel, iron, cobalt or other similar ferromagneticmaterials that are commonly used in general electronic circuitapplications.

FIG. 18 illustrates a single chamber bipolar cardiac pacemaker 10Chaving a leadwire system and showing the distal tip 50 and the distalring 52 electrodes. This is a spiral wound system where a ring coilleadwire 14 is wrapped around a tip coil leadwire 12, wherein these twoleadwires 12, 14 extend between a sealed housing 32 and the pair ofelectrodes 50, 52. There are other types of pacemaker leadwire systemsin which these two leads lay parallel to one another (known as a bifilarlead system). In FIG. 18, one can see an outer insulating sheath 58which is typically of silicone or polyurethane. This protects theleadwires 12, 14 from direct exposure to body fluid and also insulatesthe leadwires. It also has its own thermal conductive properties as willbe further described herein.

FIG. 19 is generally taken from FIG. 18 showing a typical prior artbipolar pacemaker leadwire system. Shown is the distal tip electrode 50and ring electrode 52. An insulation or insulative lead body 58 is alsoillustrated. The distal tip electrode can be passive (meaning that itcan be bent back in a “J” or shoved against myocardial tissue so that itjust rests against the tissue). A more commonly used electrode today isknown as the active fixation tip. This is an electrode where by turningthe entire center of the lead, the physicians can screw a helix intomyocardial tissue thereby firmly affixing it. A prior art activefixation electrode tip 50′ is shown in FIG. 19A. This is typically usedin conjunction with a cardiac pacemaker, an implantable defibrillator orthe like. One can see that an active fixation tip housing 60 is pressedup against the tissue to be stimulated, e.g., the myocardial tissue ofthe patient's heart. For example, this could be the septal wall betweenthe right ventricle and the left ventricle. A helix electrode assembly64 is shown in a retracted position relative to the adjacent hearttissue. Up in the pectoral pocket, the physician uses a tool to axiallytwist an assembly shaft 66, which drives a pointed tip helix screw 68into the myocardial tissue, firmly affixing it. This type of activefixation tip 50′ is becoming more popular. As can be seen, it would behighly undesirable for the active fixation helix screw 68 to heat upduring an MRI scan. Because the helix screw 68 is deeply embedded intomyocardial tissue, if excessive heating and temperature rise did occur,not only could scarring or ablation of cardiac tissue occur, but anactual cardiac wall perforation or lesion could result in sudden death.It will also be obvious to those skilled in the art that any of thefrequency impeding or diverting circuits, as shown in FIG. 4, 5, 6, 7,10 or 11, would be highly undesirable if they were located within theoverall housing 60 of the active fixation tip 50′. This is because theheat would indeed be removed from the helix screw 68, but it would betransferred into the active fixation housing 60 which rests in intimatecontact with the endocardium heart tissue. What this means is thatredirecting the MRI induced electromagnetic energy from the helix tip 68to the housing 60 simply moves the heat from one bad location to anotherbad location. Because the housing 60 is also in intimate contact withheart tissue, one would experience excessive temperature rise andresulting tissue burning, scarring or necrosis at that location as well.

Referring once again to FIG. 19, one can see that there is a ringelectrode 52 which is placed back (spaced proximally) a suitabledistance from the distal tip electrode 50. In a bipolar pacing system,the cardiac pacing pulse is produced between the tip electrode 50 andthe ring electrode 52. This electrical pulse induced into myocardialtissue produces a heartbeat. Sensing is also accomplished between thesetwo electrodes 50, 52 wherein the pacemaker can constantly monitor theelectrical activity of the heart. There are similar analogies forneurostimulators, cochlear implants and the like. There is usually apoint at which the distal electrodes, for example electrode 50, contactbody tissue or fluid for delivery of therapy involving electricalenergy. In a neurostimulator application, such as a spinal cordstimulator, small electrical currents or pulses are used to block painin the spinal nerve roots. In a urinary incontinence stimulator, adistal electrode is used to cause a muscle contraction and therebyprevent undesirable leakage of urine. In all of these examples, it wouldbe highly undesirable for excess heating defined as temperature riseabove a few degrees C., to occur particularly at the distal electrodetip(s).

In previous studies, concerns have been raised about the safety of usingmetallic structures, such as leadwires and MR scanners. Radio frequencyenergy (MHz) transmitted from the MRI scanner in order to generate theMR signal can be coupled to on the interventional device or itsassociated leads. This results in high electrical fields around theinstrument and local tissue heating. This heating tends to be mostconcentrated at the ends of the electrical structure or leads.

We can address this safety issue using the methods of the presentinvention. The concern is that distal electrodes or distal surface ringelectrodes, which directly contact body tissue, will cause local tissueburns. We need to cut/remove the electrodes from the circuit in the MHzfrequency range. In the current embodiment, this is accomplished withinductor circuit elements. In the MHz frequency range, the surface ringelectrodes are not connected to the rest of the electrical leads (FIG.9). Therefore, the ends of the leads are now buried inside of thecatheter. The concentrated, high electric fields will now be locatedinside of the catheter instead of in the tissue. This results in asignificant reduction in unwanted tissue heating.

A more effective way to “cut” or impede RF energy from the surfaceelectrodes from the rest of the circuit would be to use a parallelresonant circuit in place of the inductors in FIG. 10. This resonantcircuit could consist of an inductor in parallel with a capacitor (anL-C bandstop filter as shown in FIG. 11). If this parallel L-C circuitis tuned to the MR frequency, it will present a very high impedance atthis frequency. This will effectively cut the surface electrodes fromthe elongated leads at the MRI frequency and prevent unwanted heating.For maximal effectiveness, the L-C circuit should be shielded. For aprobe or a catheter application, with these design concepts, theelectrical end of the leads (in the MHz range) are buried inside of thecatheter body and as a result, the concentrated electric fields are alsolocated inside of the capacitor, instead of in the tissue. This resultsin a significant reduction in unwanted tissue heating. As previouslymentioned, a resonant circuit is an effective way to “cut” the surfaceelectrodes from the rest of the electrical circuit. This resonantcircuit could be an inductor in parallel with the capacitor (an L-C“tank” circuit). The L-C circuit may be placed distal to the electrodesand allowing the electrodes to be visualized. Probes and catheters oftenincorporate metallic sheaths which also assist in dissipating theunwanted energy over large surface areas. This is equivalent to theenergy dissipating surface (EDS) structures as described herein.

All of the circuit elements as described in connection with FIGS. 4through 11 are for purposes of redirecting high frequency RF energy awayfrom distal electrodes into a location that has larger thermal mass andlarger area such that the energy is not being dissipated at theconcentrated point of electrode to tissue contact. Concentrating the MRIRF energy at an electrode causes excessive temperature rise which canresult in damage to adjacent body tissues. Referring back to FIG. 3, onecan see that the leadwires 12 and 14 are embedded in the insulatingsheath of a probe, a catheter, a cardiac pacemaker lead or the like.Accordingly, if excess heat is dissipated along these leadwires, it isthen dissipated into these surrounding structures. As previouslymentioned, there is also a parasitic capacitance that's formed alongthese leadwires and the surrounding structures or insulating sheaths. Itis a feature of the present invention that any of the passive componentfrequency selective circuits can also be directly connected to energydissipating elements that are distal from the electrodes themselves.

Referring to FIG. 19 (and also FIGS. 20-22), the insulation sheath 58typically encapsulates the leadwires 12 and 14 in silicone orpolyurethane to provide strength and resistance to body fluids. Theinsulation sheath 58 has thermal conduction properties and also providesimportant electrical isolation between the leadwires 12 and 14themselves and also surrounding body fluids and tissues.

FIG. 20 is generally taken from FIG. 19 except that the twisted orcoaxial electrode wires 12 and 14 have been straightened out for betterillustration of the examples of the present invention. This is alsoanalogous to FIG. 2 for the wires of probes and catheters previouslydescribed herein. The straightened and elongated leadwires 12, 14 ofFIG. 20 are also illustrative of certain bifilar leadwire systems, whichcan also be used for pacemakers, neurostimulators and the like. In otherwords, the leadwires are not always twisted as shown in FIG. 19 as thereare certain applications where it is desirable to have the leadwires 12,14 running parallel to each other in a straight fashion. Forillustrative purposes, we will focus on the straight leadwires 12, 14 ofFIG. 20, but realize that all of these same principles to follow areequally applicable to twisted or coaxial leadwires as shown in FIG. 19.In FIG. 20, one can see that the insulation sheath 58 generally runs upto and fixates the ring electrode 52, but does not cover or encapsulateit. This is also true for the distal tip electrode 50. This is importantsuch that the electrodes are not insulated, so that they can delivertherapy and/or sense biologic signals. If they were insulated, theywould not be able to function and conduct electrical current into bodytissue. In practice, the parasitic capacitance value is quite low. Fordifferential mode induced EMFs, by electrically shorting leadwires 12and 14 together, the energy induced from an MRI system is contained intoa loop whereby it will create relatively high RF currents in leadwires12 and 14. Importantly, this loop disconnects this current flow from thedistal electrodes 50 and 52. Accordingly, this energy will be convertedto heat within leadwires 12 and 14 where it will be thermally conductedinto the insulation sheath 58 and dissipated over a much larger surfacearea. In the case where the induced EMFs are common mode, frequencyselective networks of the present invention are used to conduct the highfrequency energy to a metallic surface of the probe or catheter, such asa shield, or to an equivalent energy dissipating surface (EDS). This hasthe effect of preventing a large temperature rise at the electrode totissue interface which could be damaging to body tissue. Moreimportantly, said heat is diverted away from the distal electrodes,which make direct contact with sensitive body tissues. It is in thislocation where excessive heat dissipation can cause temperature risesthat can cause damage to body tissue and therefore, undesirable loss oftherapy or even life-threatening tissue damage. In a preferredembodiment, the parasitic capacitances or heat conductive interfacewould be replaced by passive component capacitances that are connecteddirectly to a conductive energy dissipating surface. This is a moreefficient way of diverting the energy to a point distant from the distalelectrodes and converting it to heat. By re-directing the RF and/orthermal energy to a point or an area distant from the distal electrodes,one thereby provides a high degree of protection to the sensitivejunction between the electrodes and body tissue. For example, thatjunction may be the point where a distal electrode contacts myocardialtissue and provides critically important pacing pulses. Energyconcentration at distal electrode can cause dangerous temperature rises.

FIG. 21 is generally equivalent and incorporates and embodies theconcepts previously described in FIGS. 2 through 11 herein. In FIG. 21,one can see the lead insulation 58. There is a parasitic capacitance 70and 72 which is formed between leadwires 12 and 14 and the insulationlayer 58. At high frequency, this has the desired effect of diverting orshunting high frequency MRI RF energy away from the leadwires 12 and 14thereby redirecting energy into the insulation sheath 58 where it can bedissipated over a much larger surface area with minimal temperaturerise. Series reactive elements 26 and 28, as previously described andshown in connection with FIG. 7, block, cut or impede the flow of MRIinduced RF energy to the distal tip electrode 50 and/or the distal ringelectrode 52, wherein these electrodes 50, 52 correspond respectivelywith the ring electrodes 16, 18 shown in FIGS. 2-11. These seriesfrequency selective reactances 26 and 28 are optional, but do increasethe efficacy of the present system.

Reactance 20 can be a simple capacitor as shown in FIG. 5, or it can bean L-C series trap filter as shown in FIG. 6. This tends to shortleadwires 12 and 14 together at high frequency thereby divertingundesirable high frequency energy and thereby preventing it fromreaching distal tip electrode 50 or ring electrode 52. Referring onceagain to FIG. 21, we can see high frequency RF currents I and I′. These,for example, are the RF pulsed currents induced in an elongatedimplanted lead from a 1.5 Tesla MRI system, and they would oscillateback and forth at 64 MHz thereby reversing directions, as shown, at thatfrequency. This is better understood by referring to FIG. 9. Thecurrents are cut off (as indicated at 30 in FIG. 9) and are effectivelycontained within leadwires 12 and 14. This redirects the energy that isinduced by the high frequency MR fields back into the insulation sheath58 at a point distant from the distal electrodes 50 and 52. Thisdesirably prevents the distal electrodes from overheating at their pointof contact with body tissue.

FIG. 22 is very similar to the structures shown in FIGS. 19 and 20 foractive implantable medical devices (AIMDs) such as cardiac pacemakersand the like. Shown is a selective frequency element 20 in accordancewith FIG. 6, which in this case consists of an inductor 24 in serieswith a capacitor 22 (trap filter). The component values of the inductor24 and the capacitor 22 can be selected such that they are resonant at aparticular frequency. In this case, for illustrative purposes, theyshall be resonant at 64 MHz thereby providing a low impedance shortcircuit for 1.5 Tesla MRI signals. This has the effect of diverting orshunting the energy off of leadwire 12 to the relatively large surfacearea of the ring electrode 52. The ring electrode 52 is typically ametallic structure consisting of a cylindrical ring and very highthermal conductivity. It also has, by necessity, very high electricalconductivity. Accordingly, referring once again to FIG. 22, the ringelectrode 52, by its inherent nature, becomes an energy dissipatingsurface (EDS) wherein the high frequency RF energy is diverted to it,wherein said RF energy will either be converted to heat, which will bedirected into the surrounding blood flow, or said RF energy will beharmlessly dissipated into surrounding body tissues. More specifically,for example, in the right ventricle, the distal tip electrode 50 isdesigned to be screwed into myocardial tissue in accordance with FIGS.19 and 19A. The ring electrode 52, on the other hand, is designed to beplaced back away from distal tip electrode 50 such that it actuallyfloats in the pool of blood that is flowing in the particular cardiacchamber. In an ideal situation, the wash of blood over it tends toprovide a constant cooling action through heat transfer over the ringelectrode 52 thereby dissipating undesirable heat from high frequency RFenergy harmlessly into the flowing blood (or other body fluid such aslymph in other applications). A disadvantage of this approach is that ina certain percentage of patients both the tip and the ring tend to beovergrown by tissue. Accordingly, the use of a separate energydissipating surface EDS, which is located further back from both thedistal tip and ring electrode, is desirable such that it is guaranteedto remain in the blood pool. For the energy dissipating surface EDS,which can either be the ring electrode itself or a separate energydissipating structure (EDS), it is a desirable feature that it includessome type of biomimetic coating such that tissue overgrowth isinhibited. Referring back to FIG. 21, for example, a biomimetic coating74 could be deposited all over the ring electrode to thereby inhibittissue overgrowth.

FIG. 23 is a low frequency model illustrating the concepts previouslyshown and described in connection with FIG. 7. Switches are used toillustrate the properties of the reactances at low frequency. Referringback to FIGS. 21-22, one can see that there is an insulation sheath 58or energy dissipating surface EDS. The parallel reactance element 20, asillustrated in FIGS. 4, 5 and 6, is represented respectively by switches76, 76′ and 76″. At low frequency, these reactances tend to look likevery high impedances and are therefore shown as open switches. On theother hand, the series reactances 26 and 28, as previously illustratedand described in FIGS. 7, 10 and 11, tend to look like a very lowimpedance approximating short circuits at low frequency and are thusshown respectively as closed switches 78 and 78′. Accordingly, the lowfrequency model, as illustrated in FIG. 23, is completely equivalent tothe low frequency model previously shown and described in connectionwith FIG. 8. In this case, the reactive elements 20, 26 and 28effectively disappear from the electrical model, whereby the pair ofelectrodes 12, 14 is coupled directly and respectively to the pair ofelectrodes 16 and 18.

FIG. 24 is the high frequency model of FIG. 7. In this case, thereactance element 20, previously illustrated in FIGS. 4, 5 and 7, tendsto look like a very low impedance at high frequency and therefore, isrepresented as a closed switch (or short circuit) as shown by switches76, 76′, and 76″. In FIG. 24, one can see that the series reactancecomponents 26, 28 of FIGS. 7, 10 and 11 look like very high impedances(ideally open circuits) at high frequency and are thereby shown as openswitches 78 and 78′. Therefore, the high frequency model, as illustratedin FIG. 24, is completely equivalent to the high frequency modelpreviously described and shown in connection with FIG. 9. Of course,these switches do not really physically exist and are simply a way ofillustrating the behavior of the passive component frequency selectivenetworks that are described in FIGS. 4 through 11.

FIG. 25 is a line drawing of a human heart with cardiac pacemaker dualchamber bipolar leads shown in the right ventricle RV and the rightatrium RA of a human heart 46. FIG. 25 is taken from slide number 3 froma PowerPoint presentation given at The 28th Annual Scientific Sessionsof the Heart Rhythm Society by Dr. Bruce L. Wilkoff, M. D. of theCleveland Clinic Foundation. This article was given in Session 113 onFriday, May 11, 2007 and was entitled, ICD LEAD EXTRACTION OF INFECTEDAND/OR REDUNDANT LEADS. These slides are incorporated herein byreference and will be referred to again simply as the Wilkoff reference.In FIG. 25, one can see multiple leadwires extending from an activeimplantable medical device 10 (such as a pacemaker or the like) coupledto associated electrodes, one of which comprises the distal tipventricular electrode 50 located in the right ventricular (RV) apex. Thedark shaded areas in FIG. 25 show the experience of the Cleveland Clinicand Dr. Wilkoff (who is a specialist in lead extraction), where extremetissue overgrowth and vegetation tends to occur. There are numerouscases of extracted leads where both the tip and ring electrodes havebeen overgrown and encapsulated by tissue. Referring once again to FIG.25, one can see tip electrode 50, which is located in the rightventricular apex. The shaded area encasing this electrode 50 shows thatthis area tends to become encapsulated by body tissue. An electrode 50′in the right atrium (RA) may similarly be overgrown and encapsulated bytissue, as shown by the encasing shaded area. There are other areas inthe aortic arch and venous system where leads tend to be encapsulated bybody tissue a great percentage of the time. These are shown as areas Aand B. This is particularly important to know for the present inventionsince these would be highly undesirable areas to place an energydissipating surface in accordance with the present invention. Ideallocations for energy dissipating surfaces are shown as EDS′ and EDS″ orEDS′″.

Referring once again to FIG. 25, as previously mentioned, it is veryimportant that this leadwire system does not overheat during MRIprocedures particularly at or near the distal tip electrodes and rings.If both the distal tip and ring electrode become overgrown by bodytissue, excessive overheating can cause scarring, burning or necrosis ofsaid tissues. This can result in loss of capture (loss pacing pulses)which can be life-threatening for a pacemaker dependent patient. It isalso the case where implanted leads are often abandoned (where the leadhas been permanently disconnected from the AIMD). Often times when thedevice such as a pacemaker 10 shown in FIG. 25 is changed out, forexample, due to low battery life and a new pacemaker is installed, thephysician may decide to install new leadwires at the same time.Leadwires are also abandoned for other reasons, such as a dislodged or ahigh impedance threshold. Sometimes over the course of a patientlife-time, the distal tip electrode to tissue interface increases inimpedance. This means that the new pacemaker would have to pulse at avery high voltage output level which would quickly deplete its batterylife. This is yet another example of why a physician would choose toinsert new leads. Sometimes the old leads are simply extracted. However,this is a very complicated surgical procedure which does involve risksto the patient. Fortunately, there is plenty of room in the venoussystem and in the tricuspid valve to place additional leads through thesame pathway. The physician may also choose to implant the pacemaker onthe other side. For example, if the original pacemaker was in the rightpectoral region, the physician may remove that pacemaker and choose toinstall the new pacemaker in the left pectoral region using a differentpart of the venous system to gain lead access. In either case, theabandoned leads can be very problematic during an MRI procedure. Ingeneral, abandoned leads are capped at their proximal connector pointsso that body fluids will not enter into the lead system, causeinfections and the like. However, it has been shown in the literaturethat the distal electrodes of abandoned leads can still heat up duringMRI procedures. Accordingly, a passive frequency selective circuit ofthe present invention is very useful when placed at or near the proximalelectrical contact after a pacemaker is removed and its leads aredisconnected (abandoned). For example, for an abandoned (left in thebody) lead, an energy dissipating surface EDS′″ at or near the proximallead end is an ideal place to eliminate excess energy induced by MRI inthe leadwire system.

Referring back to the article by Dr. Bruce Wilkoff, attention is drawnto slide number 2, which is an example of a lead extraction showing botha distal tip electrode and a distal ring which have been heavilyovergrown and encapsulated by body tissue. Special cutting tools wereused to free the lead so it could be extracted, so the tissue shown hereis only a small remaining portion of the mass that was originallypresent. Slide 13 is a dramatic illustration of what a larger mass ofencapsulated tissue would look like. In this case, the entire tip wascompletely surrounded, but if one looks carefully to the right, one cansee that some of the ring was still exposed. The situation is highlyvariable in that the ring is not always fully encapsulated. Slide 16 isan example of tissue removal after a pacemaker bipolar lead wasextracted. One can see at the end of the lead, the helix screw that wasaffixed to myocardial tissue. The surgeon in this photo was removing thetissue encapsulation, which completely surrounded the tip and is stillsurrounding the ring area. A blow-up of this is shown in slide 17.Again, the tissue that is still affixed to the lead has completelyencapsulated the ring, which cannot be seen. Accordingly, there is aneed for either a way to prevent the overgrowth of body tissue onto thering or to ensure that an energy dissipating surface (EDS) is locatedfar enough away from myocardial tissue to guarantee that it will remainfloating in the blood pool.

FIG. 26 illustrates an energy dissipating ring EDS which is located atsome distance “d” from both a pacemaker tip electrode 50 and a ringelectrode 52 mounted respectively at the distal ends of leadwires 12 and14. The distance “d” should be sufficient so that the energy dissipatingsurface EDS is far enough away from both the distal tip and ringelectrodes 50, 52 such that there is no heating or temperature riseassociated with the tissues that contact the tip and ring electrodes.Another advantage of moving the energy dissipating surface EDS away fromthe distal electrodes, particularly for a cardiac pacemaker application,is that there would be less tendency for the energy dissipating surfaceEDS to become encapsulated or overgrown with vegetated body tissue. Ifthe energy dissipating surface EDS, when it is disassociated at somedistance from the electrodes 50, 52, does become overgrown with bodytissue, this is not of great concern. Obviously, it would be superior tohave the EDS surface floating in freely flowing blood so that therewould be constant cooling. However, for example, if the EDS surface didtouch off to the right ventricular septum and became overgrown, the onlyeffect would be a slight heating of tissue in an area that is far awayfrom where the actual electrical stimulation and sensing is being doneby the electrodes. The ideal distance for the energy dissipating surfacedoes depend on the particular application and ranges from approximately0.1 cm to 10 cm distance from the distal electrodes.

Referring once again to FIG. 26, the energy dissipating surface EDS isshown as a cylindrical ring. It can be semi-circular, rectangular,octagonal, hexagonal or even involve semi-circles on the lead or anyother metallic or similar structure that is also thermally conductive.Literally any shape or geometry can be used as an energy dissipationsurface. It is a desirable feature of the present invention that thesurface area be relatively large so that it can efficiently dissipateheat into the surrounding blood pool and surrounding tissues that aredistanced from the electrodes. In FIG. 26, within the EDS ring, thereare electrical connections (not shown) between leadwire 12 and 14 and tothe EDS surface that embody the passive frequency selective circuitspreviously discussed in connection with FIGS. 2 through 11. The purposeof these frequency selective circuits is to remove RF induced energycaused by the RF pulsed field of MRI from leadwires 12 and 14 andredirect it to the EDS surface where it is dissipated as heat. By havinga large surface area, the heat can be dissipated without significanttemperature rise such that surrounding tissues would be burned.

In cardiac rhythm management applications, the EDS ring is ideallylocated in areas where there is freely flowing blood, lymph orequivalent body fluids which adds to the cooling. A biomimetic coating74 can be applied to the energy dissipating surface area (EDS) and/or tothe ring electrode 52 if it is used as an energy dissipating surface.This special biomimetic coating 74 provides a non-thrombogenic andanti-adhesion benefits. This coating can be comprised of a surfactantpolymer having a flexible polymeric backbone, which is linked to aplurality of hydrophobic side chains and a plurality of hydrophilic sidechains. This coating prevents the adhesion of certain plasma proteinsand platelets on the surface and hence initiation of the clottingcascade or colonization of bacteria. Biomimetic coatings also tend toprevent overgrowth or adhesion of body tissues as illustrated in theWilkoff paper. This polymer compound is described in U.S. Pat. Nos.6,759,388 and 7,276,474, the contents of both patents being incorporatedby reference herein. Additional benefits of biomimetic coatings includethe prevention of bacterial colonization and resulting infections. Itwill be obvious to those skilled in the art that other types of coatingscould be used on the EDS ring to inhibit or prevent overgrowth of bodytissue. As used herein, the term biomimetics includes all such typecoatings.

FIG. 27 is a typical quad polar neurostimulation lead system. It will beappreciated that the following discussion also applies to bipolar, hexpolar, and even 16 or 24 electrode lead systems. In FIG. 27, fourleadwires 12, 12 b, 14 a and 14 b are shown which are each directedrespectively toward an associated distal electrode 16 a, 16 b, 18 a and18 b. In this case, the electrical stimulation pulses are applied invarious combinations between the various electrodes. Unlike a cardiacpacemaker application, there is no particular ring electrode in thiscase. However, the insulation sheath 58 that surrounds the leadwires,which as mentioned could be of silicone or the like, forms a surroundingsurface, which encapsulates the leadwires.

Parasitic capacitances 70, 71, 72 and 73 are formed respectively betweeneach of the leadwires 12 a, 12 b, 14 a and 14 c and the insulatingsheath 58. As previously mentioned, these parasitic capacitances candivert high frequency pulsed RF energy from an MRI system to theinsulation sheath 58 thereby redirecting the energy so that heat will bedissipated over a larger surface area and away from the interfacebetween the distal tip electrodes 16 a, 16 b, 18 a, and 18 b and bodytissue. There is also heat that is directly dissipated off of theleadwires, which is conductively coupled into the insulation sheath 58.Again, it is desirable that this occur at a location that is spaced fromor distant from the therapy delivery electrodes 16 a, 16 b, 18 a, and 18b. This can be greatly improved by providing a passive componentfrequency selective circuit 20 which provided a very low impedance at aselected high frequency or frequencies between each of the associatedleadwires and an energy dissipating surface EDS. The energy dissipatingsurface EDS would typically either be a metallic ring or a metallicplate or even a separated metallic surface which has both the propertyof conducting the high frequency energy and also having a relativelylarge surface area for dissipating said energy into surrounding bodytissues. In a preferred embodiment, the energy dissipating surface EDSwould be placed sufficiently far back from the distal electrodes 16 a,16 b, 18 a, and 18 b so that in the associated heating of surroundingbody tissue would not have any effect on the delicateelectrode-to-tissue interface. In addition, by having an energydissipating surface (EDS) with a sufficiently large surface area, thiswill prevent a dangerously large temperature rise as it dissipatesenergy into the surrounding tissues. By controlling the temperature riseto a small amount, damage to tissue or tissue changes are thereforeavoided. The frequency selective reactances 20 are designed to present avery low impedance at selected high frequencies thereby redirectingundesirable high frequency RF energy (in the MHz range) away from theelectrodes to the insulating sheath and/or energy dissipating surface(EDS). In addition, further protection is offered by the optional seriesfrequency selective components 26, 27, 28 and 29. Typically, these canbe series inductors or they can be parallel inductor-capacitor bandstopfilters in accordance with the present invention (see FIGS. 10-11).Accordingly, substantial protection is provided such that during MRIprocedures, the distal electrodes 16 a, 16 b, 18 a, . . . 18 _(n) do notoverheat.

FIG. 28 is taken from FIG. 13 of U.S. 2008/0132987 A1 dated Jun. 5,2008, the contents of which are incorporated herein by reference.Illustrated is a side view of the human head with a deep brainstimulation electrode shaft assembly 80. At the distal end of theelectrode shaft 80 are two distal electrodes 16 and 18 (see FIG. 29)implanted into the patient's brain 82 at a selected implantation site.One or more leadwires 12, 14 (see FIG. 29A) are routed between the skin87 and the skull 88 down to a pectorally implanted AIMD (pulsegenerator) which is not shown. Referring back to FIG. 28, one can seethat an opening 84 in the skull has been made so that the electrodeshaft assembly 80 can be inserted.

FIG. 29 is taken generally from section 29-29 in FIG. 28. Shown arebipolar distal electrodes 16 and 18 at the end or tip 86 of theelectrode shaft 80. The skull is shown at 88 and the dura is shown as90. Housing 92 acts as an energy dissipating surface EDS and can behermetically sealed to protect the passive frequency selectivecomponents of the present invention from direct exposure to body fluids.

FIG. 29A is taken from section 29A-29A of FIG. 29. Shown are frequencyselective passive component circuit elements 20 which are generallytaken from FIG. 5 or 6. As previously described, these circuit elements20 could be combined with series reactance elements 26 and 28 aspreviously illustrated in FIGS. 7, 10 and 11. These have been omittedfor clarity, but would generally be placed in series with the leadwires12 and 14 and placed between frequency selective circuit elements 20 andthe distal electrodes 16, 18 (FIG. 20). Referring back to FIG. 29A,circuit elements 20 would divert high frequency RF energy induced froman MR scanner to the energy dissipating surface EDS where it would bedissipated as RF or thermal energy into the area of the skull 88 and/ordura 90. Frequency selective circuit element 20′ is also shown connectedbetween the leadwires 12 and 14. This would be effective for anydifferential mode signals that are present in the leadwires 12 and 14.In accordance with FIG. 4 of the present invention, this would redirector divert MRI induced RF energy back into leadwires 12 and 14 and awayfrom the distal electrodes 16, 18. This is an example of redirecting RFor thermal energy away from a critical tissue interface point. The skullis considered to be a relatively non-critical or less susceptible typeof body tissue to thermal injury. This is in comparison with the verythermally sensitive brain matter into which the distal tip electrodes16, 18 are implanted. It has been shown that even a temperature rise assmall as a few degrees C. can cause damage to sensitive brain matter.

FIG. 29B is generally taken from area 29B-29B of FIG. 29. Shown are thetwo bipolar electrodes 16 and 18. The frequency selective elements 20and 20′ have been moved relative to the location shown in FIG. 29A toillustrate one wrong way to approach this particular problem.Specifically, an energy dissipating surface EDS is shown mountedgenerally at a tip or other distal end portion of the probe shaft 86 inproximity to and/or direct contact with sensitive brain tissue. Thefrequency selective reactance components 20 and 20′ are coupled forredirecting the RF energy from MRI to the energy dissipating surfaceEDS, whereby heat will be dissipated by the energy dissipating surfaceEDS. In the case where it was chosen not to use an energy dissipatingsurface EDS, but simply to rely on the line-to-line frequency selectiveelement 20′, heat would still build-up in the entire distal electrodearea and thence be conducted into thermally sensitive brain tissue 82.Accordingly, the placement of the circuit elements as shown in FIG. 29Billustrates a disastrous way to place the frequency selective elementsof the present invention. Severe overheating of this distal tip wouldoccur with resulting brain damage. Reference is made to a paper given atthe 8^(th) World Congress of the National Neuromodulation Society whichwas held in conjunction with the 11^(th) Annual Meeting of the NorthAmerican Neuromodulation Society, Dec. 8-13, 2007, Acapulco, Mexico.This paper was given by Dr. Frank Shellock, Ph. D. and was entitled, MRIISSUES FOR NEUROMODULATION DEVICES.

Shellock slide 31 shows X-ray views of the placement of deep brainstimulator electrodes into the skull and brain of a human patient. Thereis also an X-ray view showing the placement of the AIMDs and tunneledleadwires that are associated with the deep brain stimulationelectrodes. Slide number 35 shows an extensive thermally induced lesionshown in white with a red arrow to it. This was representative of twopatients that inadvertently received MRI wherein their deep brainstimulators overheated and caused extensive thermal injury to the brain.Both patients were severely disabled.

In summary, the configuration as illustrated in FIG. 29A is highlydesirable as compared to the configuration as illustrated in FIG. 29B.

Referring once again to the Shellock paper, one can see that the deepbrain stimulator involved multiple electrodes. In FIG. 29 one can seethat there are only two electrodes 16 and 18. This is a way ofillustrating that with real time MRI guidance, the physician can muchmore accurately place the electrodes into the exact area of the brain,which needs to be electrically stimulated (for example, to controlParkinson's tremor, Turret's Syndrome or the like). What is typicallydone is that precise MR imaging is performed prior to electrodeimplantation which is referenced to fiducial marks that's placed on theskin in several locations outside of the patient's skull. The patient'shead is first shaved, then these marks are placed and then the MRI isperformed. Then when the patient enters the operating room, a fixture isliterally screwed to the patient's head at these fiducial marks. Thisfixture contains a bore into which the various drilling and electrodeimplanting tools are located. Because of the need for all of thismechanical fixturing, tolerances are involved. This means that by thetime the electrodes are implanted in the brain, they may be not in theprecise locations as desired. Accordingly, extra electrodes are insertedwhich involves more leadwires than are really necessary. The patient isusually awake during parts of this procedure wherein the physician willuse trial and error to stimulate various electrode pairs until thedesired result is achieved. In contrast, the present invention minimizesthe need for all these extra electrodes and extra wiring. This isbecause by eliminating the potential for the distal electrodes tooverheat and damage brain tissue, this entire procedure can be doneunder real time MRI imaging. In other words, the physician can bewatching the MRI images in real time as he precisely guides theelectrodes to the exact anatomy of the brain that he wishes tostimulate.

FIG. 30 is a hermetically sealed package consisting of a passive distaltip electrode 50 which is designed to be in intimate contact with bodytissue, such as inside the right atrium of the heart. A hermetic seal isformed at laser weld 94 as shown between the tip electrode 50 and ametallic ring 96. Gold brazes 98 are used to separate the metallic ring96 from the energy dissipating surface EDS by use of an interveninginsulator 100. This insulator 100 could typically be of alumina ceramic,other types of ceramic, glass, sapphire or the like. The energydissipating surface EDS is typically gold brazed to the other side ofthe insulator 100 as shown. An inductor 26′, such as an inductor chip inaccordance with FIG. 10, is shown connected between the distal tipelectrode 50 and a conductive lead 102 which is attached as by laserwelds 104 to the end of the leadwire 12 extending through the body tothe AIMD. As shown, the lead 102 protrudes through a hermetic sealassembly 106 formed by a metallic flange 108 which is typically oftitanium or platinum or the like. The flange 108 is hermeticallyattached to the lead 102 as by gold brazes 110, and is typically laserwelded as shown at 112 to a proximal end of the energy dissipatingsurface EDS.

FIG. 31 is taken generally from the housing of FIG. 30. It is importantthat the electrical insulating material 100 either be of very lowthermal conductivity or have a relatively long length “L” as shown. Thereason for this is that the thermal energy that is developed in theenergy dissipating surface EDS must not be allowed to reach the distaltip electrode 50 as shown in FIG. 30 where heat could cause damage tothe adjacent tissue.

The energy dissipating surface EDS is typically of biocompatible metals,such as titanium, platinum or the like. It is important that the energydissipating surface be both electrically conductive and thermallyconductive so that it can transfer RF and thermal energy into body fluidor tissue. The energy dissipating surface EDS can be roughened or evencorrugated or bellowed as shown in FIG. 31A to increase its surface areaand therefore its energy dissipating properties into surrounding bodyfluids or body tissue.

In accordance with FIG. 5, capacitive elements 22 and 22′ shown in FIG.30 are designed to act as a low impedance at higher frequencies.Electrical connections 114 and 116 (FIG. 30) couple the capacitor 22 tothe energy dissipating surface EDS, whereas electrical connections 118and 120 couple the capacitor 22′ to the energy dissipating surface EDS.This forms a broad band low pass filter wherein the inductor 26′ acts incooperation with the capacitive elements 22 and 22′. The presence of theinductor element 26′ is not required; however, it does enhance theperformance of the capacitor elements 22 and 22′. Capacitor elements 22and 22′ are typical off-the-shelf commercial monolithic ceramiccapacitors (MLCCs). These are better illustrated in FIG. 33A.

There is an advantage in the present invention in using a capacitor forthe selective frequency element 20 as shown in FIG. 5. The capacitortends to act as a broadband filter which will attenuate a range of MRIfrequencies. For example, placement of an effective capacitor 20 couldattenuate 64 megahertz, 128 megahertz and higher MRI frequencies.However, if one were to use an L-C series trap filter as shown in FIG. 6for the variable frequency element 20, then this would only be effectiveat one MRI frequency, for example, 64 megahertz only. Of course, asalready been disclosed herein, one could use multiple L-C trap filters.However, in a preferred embodiment the use of a capacitor as illustratedin FIG. 5 is desirable because with a single component, one canattenuate a broad range of MRI frequencies.

The schematic diagram for the circuitry of FIG. 30 is shown in FIG. 32.Capacitors 22 and 22′ are actually in parallel and act as a singlecapacitive element. The reason for multiple capacitors is to obtain ahigh enough total capacitance value so that the capacitive reactance isvery low at the frequency of interest (for example, 64 MHz for a 1.5 TMR system).

An alternative capacitor 22″ for use in the circuit of FIG. 32 is knownas a unipolar feedthrough capacitor is shown in FIG. 33B. It has outsidediameter and inside diameter termination surfaces 124 and 122 forelectrical contact. Feedthrough capacitors can be unipolar ormultipolar. These are completely described in the prior art; forexample, refer to U.S. Pat. No. 7,363,090, particularly FIGS. 3, 5, 29through 31, and 39. See also U.S. Pat. Nos. 4,424,551; 5,333,095; and6,765,779.

FIG. 34 is similar to FIG. 30 (using common reference symbols) exceptthat the inductor element 26′ is wire wound around a non-ferromagneticmandrel 126 (formed from a material such as a ceramic or plastic). Thistype of wound inductor 26′ has much higher current handling capabilityas compared to the inductor chip of FIG. 30. The inductor chip of FIG.30 can be fabricated from a variety of shapes including Wheeler'sspirals and the like. Refer to U.S. Patent Publication No. 2007-0112398A1, FIG. 83. A Wheeler's spiral is illustrated in FIGS. 42 and 43 ofU.S. Patent Application No. 60/767,484. A composite inductor isillustrated in FIG. 46 of U.S. Patent Application No. 60/767,484. Alsorefer to FIGS. 70 and 71 of U.S. Patent Application No. 61/038,382.These inductors can be manufactured by a number of printing techniquesincluding lithographic or copper clouting and etching. However, thisresults in relatively thin and high resistivity inductor traces.

It is important that the inductor element 26′ of the present inventionbe able to handle substantially high currents when it is in series withthe lead 102. The reason for this has to do with either ICD applicationsfor shock electrodes or automatic external defibrillation (AED) events.AEDs have become very popular in government buildings, hospitals,hotels, and many other public places. When the external defibrillatorpaddles are placed over the chest of a cardiac pacemaker patient, thehigh voltage that propagates through body tissue can induce powerfulcurrents in implanted leads. Accordingly, the inductor 26′ of thepresent invention has to be designed to handle fairly high current (ashigh as the 4 to 8 amp range in short bursts). The wire wound inductor26′ of FIG. 34 has wire of a larger cross-sectional area and istherefore a higher current handling inductor and is therefore apreferred embodiment.

FIG. 35 illustrates an entirely different approach for the diverting ofRF energy away from the electrode tip DT to the energy dissipationsurface EDS. Shown are electrical connections 128, 130 between a firstinductor 26′ and the distal tip electrode assembly 50. The other end ofthe first inductor 26′ is connected to a second inductor 26′″ which isin turn electrically connected at 132 to the leadwire 102. The capacitor22 is connected between the junction of the two inductors 26′ and 26′″at electrical connection 134. The other end of the capacitor iselectrically connected at 136 to the energy dissipating surface EDS. Aninsulating sleeve (not shown) can be used to ensure that the capacitortermination and electrical connection 134 does not inadvertently makecontact (short out) with the energy dissipating surface EDS. As shown,this connection is made adjacent to the insulator 100 so there is nochance for such shorting out.

The electrical schematic for FIG. 35 is shown in FIG. 36. In accordancewith FIG. 7, this forms what is known in the art as a low pass filter(in this example, a T filter), which tends to enhance the filteringperformance by directing more of the RF energy to the energy dissipatingsurface EDS. As previously mentioned, a single or multi-element low passfilter would attenuate a broad range of MRI frequencies and would be anadvantage in the present invention for that reason.

The various types of low pass filters are more thoroughly shown in FIGS.37 and 38 which compares the filtering efficiency measured asattenuation in dB with increasing numbers of filter elements. Shown area single element low pass filter consisting of the capacitor 22 or aninductor 26, an L filter which consists of an inductor 26 and acapacitor 22, a T filter as shown in FIGS. 35-37, a Pi filter (FIG. 38),an LL filter (FIG. 38) or an “n” element filter (FIG. 37). FIG. 37 showsthe general response curves of these types of filters in attenuationversus frequency. The schematics for these various filters, which arecorrelated to the curves in FIG. 37, are shown on FIG. 38. As oneincreases the number of filter elements, the ability to attenuate orblock high frequency signals from reaching a distal electrode isimproved. Referring once again to FIG. 37, for example, one can see,that for a particular value of a single element capacitive filter, theattenuation for a 1.5 Tesla MRI system operating at 64 MHz is only about12 dB. This means that a certain amount of the RF energy would stillreach the distal tip electrode. Now compare this to the T filter ofFIGS. 35-37, where one can see that there is in excess of 45 dB ofattenuation. In this case, an insignificant amount of RF energy from theRF pulsed frequency of the MRI, would reach the distal electrode.Accordingly, one preferred embodiment of the present invention is that acapacitor combined with one or more inductors would be an optimalconfiguration. As the number of elements increases, the filteringefficiency improves. When the filtering efficiency improves, this meansthat less and less RF energy will reach the distal tip.

FIG. 39 illustrates a schematic diagram of a series inductor24-capacitor 22 filter which is commonly known in the industry as a trapfilter. The trap filter was previously described in connection with FIG.6. Referring once again to FIG. 39, there is a particular frequency fora trap filter when the capacitive reactance becomes equal and oppositeto the inductive reactance. At this single frequency, the capacitivereactance and the inductive reactance cancel each other out to zero. Atthis point, all one has left is the residual resistance 138. If oneselects high quality factor (Q) components, meaning that they are verylow in resistance, then the trap filter of FIG. 39 ideally tends to looklike a short circuit at its resonant frequency F_(r) between points Aand B which may comprises connections respectively to a pair ofleadwires 12 and 14. FIG. 39A gives the resonant frequency equationwhere F_(r), in this case, was measured in hertz. FIG. 9 shows theeffect of a short circuit 30 between leadwires 12 and 14. Referring onceagain to FIG. 39, it is important that the amount of resistance R becontrolled. This is better understood by referring to FIG. 40.

FIG. 40 illustrates the impedance Z in ohms versus frequency of theseries resonant L-C trap filter of FIG. 39. As one can see, theimpedance is quite high until one reaches the frequency of resonanceF_(r). At this point, the impedance of the series L-C trap goes very low(nearly zero ohms). For frequencies above or below resonance F_(r),depending on the selection of component values and their quality factor(Q), the impedance can be as high as 100 to 1000 or even 10,000 ohms orgreater. At resonance, the impedance tries to go to zero and is limitedonly be the amount of parasitic resistance 138 (FIG. 39) that isgenerally composed of resistance from the inductor 24 and also theequivalent series resistance from the electrode plates of the capacitor22. There is a trade off in proper selection of the components thatcontrols what is known as the 3 dB bandwidth. If the resistance isextremely small, then the 3 dB bandwidth will be narrower. However, thismakes the trap filter more difficult to manufacture. Accordingly, the 3dB bandwidth and the resistive element R are preferably selected so thatit is convenient to manufacture the filter and tune it to, for example,64 MHz while at the same time providing a very low impedance R at theresonant frequency. For an ideal L-C series resonant trap filter,wherein ideal would mean that the resistance R would be zero, then theimpedance at resonance would be zero ohms. However, in this case, the 3dB bandwidth would be so narrow that it would be nearly impossible tomanufacture. Accordingly, some amount of resistance R is in factdesirable.

As previously mentioned, there is a disadvantage to use of the L-C trapfilter as shown in FIG. 6. That is, it is really only effective forattenuating the one MRI frequency (for example, 64 megahertz for a 1.5megahertz scanner). Accordingly, when the AIMD manufacturer would applyfor their FDA conditional labeling, they could only claim compliancewith 1.5 Tesla MRI scanners. However, the L-C trap filter of FIG. 6 alsooffers a very important advantage in that it offers a very high degreeof attenuation at this one selected frequency and is also highlyvolumetrically efficient. Accordingly, there is a trade-off here. Whenone uses a broadband low pass filter, a broad range of frequencies isattenuated at the cost of increased size and complexity (an additionalnumber of components). An L-C trap filter such as shown in FIG. 6 ismore of a “rifle-shot” approach wherein only one selected frequency isattenuated. In physics, this is more efficient and tends to make thecomponents smaller.

FIG. 41 illustrates yet another method of decoupling RF signals fromleadwire 12. Referring back to FIGS. 30 through 38, all of theaforementioned decoupling techniques involve broad band low passfiltering. The advantage with these is that they would be applicable toa wide range of MRI machines including 0.5, 1.5, 3.0, 5.4 Tesla and soon. In other words, these broad band EMI filters would attenuate a broadrange of RF frequencies. In FIG. 41, one can see that there are twodiscrete L-C trap filters. The first trap filter consists of inductor 24and capacitor 22 acting in series, and the second trap filter consistsof inductor 24′ and capacitor 22′ operating in series. This is bestunderstood by referring to the schematic of FIG. 42 which shows theseries connection of 24, 22 from the lead 102 to the energy dissipatingsurface EDS. Inductor 24′ and capacitor 22′ are also connected in seriesfrom the lead 102 to the energy dissipating surface EDS.

In FIG. 41, one can see that an electrical connection 128 is madebetween the distal tip electrode 50 and inductor chip 24. Inductor chip24 is then electrically connected via electrical connection material 130to monolithic chip capacitor (MLCC) capacitor 22. The other end of thechip capacitor 22 is electrically connected at 134 to the energydissipating surface EDS. Inductor 24′ is also connected to the distaltip electrode 50 by material 136. The other end of inductor 24′ isconnected in series at 132 with capacitor 22′. The other end ofcapacitor 22′ is electrically connected at 140 to the energy dissipatingsurface EDS. In this way, the two trap filters are connected in parallelbetween the lead 102 and the energy dissipating surface EDS as shown inthe schematic diagram of FIG. 42.

FIG. 43A illustrates a typical chip inductor 24, 24′ which can be usedin FIG. 41.

FIG. 43B is a typical prior art MLCC chip capacitor 22, 22′ which canalso be used in conjunction with the package shown in FIG. 41.

FIG. 44 is a graph of impedance versus frequency showing the impedancein ohms for the L-C trap filter elements that were previously describedin FIGS. 41 and 42. By carefully selecting the component values 22 and24 and also 22′ and 24′, one can select the frequencies at which the two(or more) L-C trap filters will self-resonate. In the present example,the first trap filter including components 22 and 24 has been selectedto resonate at 64 MHz, and the second trap filter including element 22′and 24′ has been selected to resonate at 128 MHz.

Referring once again to FIG. 44, one can see that we now effectivelyhave dual trap filters which tend to short out leadwire 102, 12 at twodifferent frequencies. In this case, by example, the first trap filterresonates at 64 MHz, which is the RF pulsed frequency of a 1.5 Tesla MRIsystem. The second trap filter, which has resonant frequency F_(r2) at128 MHz, is designed to attenuate the RF pulsed signals from a 3 TeslaMRI frequency. It will be appreciated that a multiplicity of trapfilters can be used depending on how many different types of MRI systemsthat one wants to have compatibility with for an implanted lead andelectrode. The method of selecting the resonant frequency was alreadydescribed in FIG. 39A and is applicable to FIG. 44. Referring once againto FIG. 44, one will note that except at the resonant frequency F_(r1)and F_(r2), the impedance of the trap filter is very high. This is veryimportant so that low frequencies are not attenuated. Accordingly, usinga cardiac pacemaker application as an example, pacing pulses would befree to pass and also low frequency biologic signals, such as those thatare produced by the heart. It is very important that pacemaker sensingand pacemaker pacing can occur while at the same time, high frequencyenergy, for example, that from the RF pulsed frequency of an MR systemcan be diverted to an appropriate energy dissipating surface EDS.

FIG. 45 illustrates a typical active implantable medical device bipolarleadwire system. On the left is shown a distal tip electrode 50 and adistal ring electrode 52. The energy dissipating surface EDS of thepresent invention is shown along with coaxial leadwires 12 and 14 whichwould be connected to the AIMD. These could be endocardial or epicardialin accordance with the prior art.

FIG. 46 is a blown up sectional view generally taken from section 46-46from FIG. 45. In FIG. 46, one can see that there is an energydissipating surface EDS which is enclosed at both ends by two hermeticseal flanges or flange assemblies each consisting of a flange 108, aninsulator 142 and gold brazes 110. This is designed to be laser weldedas at 112 into the metallic energy dissipating surface EDS as shown. Abipolar feedthrough capacitor 22′″ is shown in cross-section in FIG. 46where the two leadwires 12 and 14 pass through it. The feedthroughcapacitor 22′″ is a very efficient broad band filter which would tend todecouple high frequency signals such as 64 MHz (1.5 Tesla) and 128 MHz(3 Tesla) from the leadwires 12, 14 to the energy dissipating surfaceEDS in accordance with the present invention. Each leadwire 12 and 14may additionally include the frequency selective reactances 26 and 28(as previously shown and described in FIGS. 7, 10 and 11).

The bipolar feedthrough capacitor 22′″ is illustrated in isometric viewin FIG. 47. Shown is an outside diameter termination surface 124 whichis electrically and thermally connected to the inside diameter of theenergy dissipating surface EDS of FIG. 42, as by electrical connection144 (FIG. 46). Also shown, are inside termination surfaces 122 and 122′located on the inside diameter of two feedthrough capacitor ID holes forelectrical connection at 146 and 146′ (FIG. 46) between leadwires 12 and14, respectively to the feedthrough capacitor termination surfaces 122and 122′, respectively. The use of a feedthrough capacitor in this casemakes for a truly broadband performance. As MR systems continue toevolve in their static magnetic field strength, the RF pulse frequenciesgo higher and higher. For example, for a 10 Tesla scanner, the RF pulsefrequency is 426.5 megahertz. Prior art MLCC chip capacitors haveinternal inductance and tend to self-resonate at frequencies around 400megahertz or above. Accordingly, the use of a feedthrough capacitoraccommodates much higher frequency MRI systems.

Referring once again to FIG. 26 and FIG. 29, one can understand why theenergy dissipating surface EDS of FIG. 45 has been moved back a suitabledistance “d” from the distal tip electrode 50 and the distal ringelectrode 52. This is because of the tendency for distal tip 50 and ringelectrodes 52 to become completely embedded or encapsulated with bodytissue. In other words, one cannot guarantee that the distal ringelectrode 52 will always be freely floating in the blood pool, forexample, of the right ventricle or the right atrium. Referring onceagain to FIG. 25, one can see shaded areas where tissue encapsulationtends to be the greatest. An ideal location for the energy dissipatingsurface EDS, as described in FIG. 45, is shown as EDS′ in FIG. 25. Thisguarantees that the energy dissipating surface is placed generally intothe area of the right ventricle that is free of traebuclar tissue andwhere there is always freely flowing blood. Of course, this isparticularly important for cardiac rhythm management applicationswherein pacemakers and implantable defibrillators are commonly used. Forimplantable neurostimulators, generally, these are not placed in areaswhere there is freely flowing blood. However, it is still important inthese cases that the energy dissipating surface be a sufficiently largeenough distance from the associated electrode(s) so that if there isadjacent tissue heating, it does not affect the delicate interfacebetween the electrodes and surrounding body tissue. This would beparticularly important, for example, in a deep brain stimulator. Asshown in FIG. 29, for example, an ideal location for the energydissipating surface would be either at the skull or subdural (slightlybelow the skull). In this case, the deep brain stimulation electrodewould protrude down into the brain tissue below the energy dissipatingsurface EDS. In this case, the RF energy and/or heat would be dissipatedover a relatively large surface area well away from the very heatsensitive and delicate brain tissues 82. For a spinal cord stimulator,there is generally freely flowing spinal fluid which can act as acooling agent as well. In this case, it is desirable to have the EDSsurface, again, spaced at some distance from the therapy deliveryelectrode such that cooling effectively takes place within the cerebralspinal fluid. See U.S. Publication Nos. US 2008-0132987 A1 and US2007-0112398 A1, which are incorporated by reference herein. In somecases, the separation distance can be quite small, for example on theopposite surface of a paddle electrode as shown in FIGS. 64, 65 and 66.

FIG. 48 is a schematic diagram of the energy dissipating surfaceassembly previously described in FIGS. 45 and 46. From FIG. 48, one cansee that the passive frequency selective elements 22 and 22′ could bereplaced by any of the circuits previously described in FIGS. 4 through11 as element 20.

FIG. 49 illustrates a bipolar lead of the present invention with distaltip and ring electrodes 16, 18 shown distally at a suitable distance dfrom a energy dissipation surface (EDS) such that energy dissipation inthe EDS would not cause a temperature rise at the distal electrodes.Shown is a capacitor 22 connected between the leadwires 12 and 14. Alsoshown are a pair of bandstop filters 26″ and 28″ as previouslyillustrated in FIG. 11. In fact, the passive component networkconfiguration of FIG. 49 is identical to that previously illustrated inFIG. 11, wherein the frequency selective element 20 is a capacitor 22.Referring once again to FIG. 49, one can see that the capacitor element22 acts as a high frequency energy diverter. This works in cooperationwith the two bandstop filter elements 26″ and 28″ which act as energyimpeders at a selected MRI frequency. Accordingly, high frequency energythat is induced on the leadwires 12 and 14 is converted to RFcirculation currents I₁ and I₂. I₁ and I₂ are shown in oppositedirections to illustrate, for example, for a 1.5 Tesla MRI system, thatthese oscillate back at 64 million times per second. This creates agreat deal of current in the associated leadwires to the right (asviewed in FIG. 49) of the diverting element 22. This causes heat to bedissipated in the leadwires 12 and 14 into the energy dissipatingsurface EDS such as the overall insulation sheath or shield of theprobe, catheter or implanted device as shown.

FIG. 50 is very similar to FIG. 49 except that diverting element 20 hadbeen replaced by a pair of capacitor elements 22 and 22′ which connectfrom leadwires 104 and 104′ respectively to an electromagnetic shield oran energy dissipating surface EDS. It is a desirable property of thepresent invention that the EDS surface be highly thermally conductive,have relatively high surface area for efficient transfer of RF or heatenergy into surrounding fluids and body tissue and also be electricallyconductive at RF frequencies.

The bandstop filters 26″ and 28″ of FIG. 50 look like a very highimpedance (ideally an infinite impedance) at the resonant frequency.This has the effect of disconnecting the distal electrodes at these highfrequencies from the leadwires 12 and 14. These work in conjunction withthe low pass filter elements 22 and 22′ which act as a way to divert thehigh frequency energy to the energy dissipating surface EDS. Aspreviously mentioned, the low pass filter elements 22 and 22′ canconsist of any of the low pass filters as previously described in FIGS.37 and 38 or the L-C trap filter as previously described in FIGS. 39,39A, 40, 41 and 44. A high frequency model of FIG. 50 is illustrated inFIG. 9 wherein the leadwires are effectively shorted together to anenergy dissipating surface EDS and the distal electrodes 16 and 18 havebeen effectively disconnected (in this case, by the bandstop filterelements) from the electrodes. For a more complete description ofbandstop filter elements and their design and operation, refer to U.S.Pat. No. 7,363,090.

FIG. 51 illustrates an exemplary bandstop filter 26″ or 28″ consistingof a parallel inductor L and capacitor C (as previously shown anddescribed herein) with nonlinear circuit elements such as diodes 148 and150 placed in parallel therewith. These diodes 148, 150 are oriented inwhat is known in the prior art as a back-to-back configuration.Nonlinear circuit elements (such as diodes) can allow the device to be“switched” between different modes of operation. The diode elements 148,150, as illustrated in FIG. 51, can be placed in parallel with eachother, and with any of the frequency selective circuit elements aspreviously described in FIGS. 4 through 11. For example, referring toFIG. 5, the diode elements 148 and 150 could be placed in parallel withthe capacitive element 22. Referring to FIG. 10, two diode elements 148,150 could also be placed in parallel with each of the inductor elements26′ and 28′. As previously discussed, automatic external defibrillators(AEDs) have become very popular in the patient environment. Accordingly,implanted leads must be able to withstand very high pulsed currents.These pulse currents can range anywhere from 1 to 8 amps. It is also afeature of the present invention that the passive frequency selectivecomponents be very small in size. In order for an inductor element L tobe able to handle 1 to 8 amps, it would have to be exceedingly large.However, by using very small diode elements 148 and 150, one can havethe circuits switched to a different state. That is, when a highvoltage, such as that from an AED appears, the diodes would forward biasthereby temporarily shorting out the bandstop filter 26″ or 28″consisting of the parallel combination of inductor L and capacitor C(FIG. 51.). Thereby the correspondingly high AED induced currents wouldbe diverted away from the relatively sensitive (small) passive elementsL and C in such a way that they not be harmed.

FIG. 52 is nearly identical to FIG. 50 except that transient voltagesuppressors 152 and 152′ have been added respectively in parallel withthe bandstop filter elements 26″ and 28″. Transient voltage suppressorsare nonlinear circuit elements which operate in much the same fashion aspreviously described for the back-to-back diodes 148 and 150 of FIG. 51.This family includes diodes, zener diodes, Transorbs™, Transguard®,metal oxide varistors, Z_(n)0 varisters, and other similar nonlinearcircuit elements. The purpose of the transient voltage suppressors 152and 152′ in FIG. 52 is to bypass any high voltage induced currents suchthat these currents not flow through the relatively sensitive bandstoppassive component inductor and capacitor elements.

FIG. 53 illustrates a general filter element 160 which can berepresentative of any of the filters previously described. The filterelement 160 of FIG. 53 is shown disposed between an electricalconnection to an energy dissipating surface EDS as illustrated. Thefilter is shown connected to a proximal end of a leadwire 12 or the likewith dashed lines, and connected to a distal end electrode 16 showncoupled to the leadwire 12 or the like with dashed lines. The reason forthe dashed lines is an indication that the filter 160 can be placedanywhere between the distal end and the proximal end of the leadwire 12.The filter 160 and energy dissipating surface EDS could be located nearthe distal end, at the distal end, at a distal ring electrode 52 or neara distal ring electrode 52 such that it would float in the blood pool.The filter 160 can also be placed at or near the proximal end, or at anypoint between the distal and proximal ends.

In particular, the filter and associated energy dissipating surface EDScould be all the way at the proximal end of an abandoned lead. Leads areoften abandoned in application for various reasons. Sometimes the leadbecomes slightly dislodged, for example, from cardiac tissue such thatthe pacing threshold increases or is lost. Sometimes lead insulationbecomes abraded and/or the leadwire itself is broken. Removing leadsonce they've been in the body for a long time can be very difficult asportions of the lead tend to become overgrown by body tissue. One isagain referred to the article entitled, ICD EXTRACTIONINFECTED/REDUNDANT LEADS EVERYDAY CLINICAL PRACTICE by Dr. BruceWilkoff. When one looks at the photographs of the extracted leads, onecan see that they are very often substantially overgrown with tissue.Therefore, it is common practice to simply abandon leads.

In the prior art, the abandoned lead is simply capped such that bodyfluid will not enter it. This cap is nothing more than an insulativecap. However, it is also well known in the literature that abandonedleads can be quite dangerous in an MR scanning situation. High energyelectromagnetic fields from the RF pulsed energy of a scannerintensifies at the ends of implanted leads. Because they are abandonedor capped at one end, this creates a reflection situation whereby all ofthe intense energy has no way to escape the lead except at the distalelectrode end. This is the worst case situation because the distalelectrode makes intimate contact with body tissue. For example, if thetissue was myocardial tissue, one runs a severe risk of creating burningor lesions in the heart. In the case of a deep brain stimulator, oneruns the risk of causing deep lesions within the brain. In an abandonedlead, therefore, it is much more desirable that energy be dissipated ator near the proximal end as opposed to the distal end where there aresensitive body tissues involved. In general, active implantable medicaldevices are implanted in muscle or in fat tissues, for example, in thepectoral areas which are not so heat sensitive, but more importantly,are not implanted in an organ, whose function could be compromised.Accordingly, it is a feature of the present invention that any of thefilter networks, as previously described herein, including those asshown in FIGS. 4 through 11, could be incorporated in a cap structure tobe attached to the proximal end of the leadwire wherein such said capstructure includes an energy dissipating surface. For a furtherdescription of the problem and the need to provide a cap for abandonedleads, one is referred to U.S. Pat. No. 6,985,775.

FIG. 54 shows an energy dissipating surface EDS in a relatively fixedlocation along the length of a leadwire 12. In accordance with thepresent invention, the energy dissipating surface EDS is placed asuitable distance d from a distal electrode 16 such that energydissipation in the area of the EDS surface will not cause tissueoverheating at or near the distal electrode 16. Also shown is afrequency impeding element 26 which can be moved to various locationsalong the length of the leadwire 12 as indicated by the multipledashed-line boxes 26. For example, impeding element 26 could be placednear the energy dissipating surface EDS, or it could be moved toward thedistal electrode 16 at any one of several successive locations. Theimpeding element 26 such as a bandstop filter 26″ or a series inductorwill still work in conjunction with the diverting element 20 at any ofthese various locations. In fact, this can be an advantage in thepresent invention in order to make the distal tip electrode 16 and itsassociated leadwire 12 within the distance “d” smaller in diameter. Ingeneral, most leads for cardiovascular applications are restricted tothe six French (0.079 inches in diameter) region. This can beproblematic for a biventricular implant where the endocardial electrodemust be threaded through the venous system and then into the coronarysinus and through the great cardiac vein to one of many branch vesselswhich are outside of the left ventricle. These branch vessels tend to bevery small in diameter and very difficult to navigate, particularly fora large lead (size four French or smaller would be ideal). There is alsoa similar need for certain spinal cord and deep brain stimulators whichmust embody electrodes that are very small in diameter. Referring backto FIG. 54, one can see that by having a relatively large diverterelement 20 associated with a energy dissipating surface EDS that islocated at a distance d from the distal electrode, one can then downsizethe diameter of the wiring along the length of distance d. By puttingthe frequency impeding element such as any one of the elements 26, 26′and/or 26″, one can make this single component smaller than multiplecomponents. Accordingly, frequency impeding elements do not have to bein direct physical proximity to diverting frequency selective elements20. As taught in FIGS. 4, 5, 6, 37 and 38, the diverting element 20 canconsist not only in a capacitor or an L-C resonant trap filter, but alsocould include a variety of low pass filters. Referring to FIG. 37, forexample, one could see that an L section low pass filter is identical tothe filter described in FIG. 54, wherein element 26 represents theinductor element and element 20 represents the capacitor element.Referring once again to FIG. 54, one can incorporate a T-type filterwhich embodies two inductor elements. In this embodiment, the left handinductor element 26 would be to the left of the frequency divertingelement 20 and a second inductor (not shown) would be located to theright of the diverter element 20 (see FIG. 37). This right hand inductorcould be located in close physical proximity to the diverter element 20,or it could also be moved away as was described for the left handinductor element at various locations as shown in FIG. 54.

Referring back to FIG. 54, it should be noted that the variableimpedance element 20 can be monolithic ceramic (MLCC) capacitors,ceramic feedthrough capacitors, or other types of capacitive circuitcomponents. In addition, the frequency selective element 20 can be aparasitic or distributive capacitor wherein the capacitance is formedthrough relatively high-dielectric materials between leadwires orelectrodes in an energy dissipating surface.

FIG. 55 illustrates a type of probe or catheter 10 which is typicallyused to both map and ablate the inside of cardiac chambers to eliminateor control certain types of arrhythmias. For example, in a patient withuncontrollable atrial fibrillation, this type of probe or catheter 10would be inserted so that electrical mapping, between bipolar distalelectrodes 16 and 162 or between electrodes 18 and 18′, could beperformed to isolate and locate those areas from which the sporadicelectrical activity is occurring. For example, this might be around apulmonary vein. Reference is made to U.S. Pat. No. 7,155,271 for a morecomplete description of this type of need and procedure. After the areasthat need to be ablated are located, the surgeon can apply RF ablationenergy at a distal ablation electrode 162. This has the effect ofburning the inside of cardiac tissue creating a scar which will isolatethis area of erratic electrical activity. The goal here is to complete ascar structure such that the atrial fibrillation is terminated.Unfortunately, in the prior art, this procedure is done using real-timeX-ray, fluoroscopy or other types of guidance, which does not adequatelyvisualize soft tissue. Accordingly, the surgeon is working pretty muchblind as the scars forming cannot be seen in real time. As explained inU.S. Pat. No. 7,155,271, it would be a great advantage if suchprocedures could be performed during real time MRI guidance. The problemis the MRI RF energy induced into the ablation catheter could causeoverheating and sporadic formation of scar tissue at the wrong timeand/or in the wrong location. In FIG. 55, one can see that there is anovel energy dissipating surface EDS of the present invention. This EDSsurface is located at a distance “d” back from the distal tip such thatthe energy dissipating surface will redirect energy away from both theelectrical sensing electrodes 16, 18 and the RF ablation electrode 162where they cannot overheat at inappropriate times. Frequency selectivepassive components (not shown), in accordance with the presentinvention, are connected in series with the leadwires, or from theinside of the energy dissipating surface EDS to the various leadwires12, 14 and 164. These are the circuits that have generally beendescribed in FIGS. 4 through 11 herein. For simplicity, they have notbeen shown in FIG. 54, but should be obvious to one skilled in the artfrom the previous drawings. In other words, the RF ablation electrodetip 162 will only overheat when the surgeon decides to activate the RFcircuitry to deliberately form the scar tissue.

The energy dissipating surface EDS may include some materials or antennastructures that are readily visualized during active MRI guidance. Thismay be important so that a physician can ensure that if the probe orcatheter is manipulated that the EDS surface not rest against the insideof, for example, the atrial septum. This is the area that is dissipatingRF energy and heat during the active MRI. If the surface area of thisEDS surface is sufficiently large so that very little temperature risewould occur, it would not matter if the EDS surface touched off against,for example, the inside wall of the cardiac septal wall. However, if theEDS surface was relatively small, then substantial temperature risecould occur if it was not kept within the freely flowing blood stream.In this case, it would be important that the physician be able tovisualize the EDS surface and the MRI images so that it not be allowedto rest inappropriately against sensitive tissues on the inside of theatrium and cause inadvertent scar tissue or ablation to occur. Referringonce again to FIG. 55, one can see that the ablation electrode 162 isconnected to an RF ablation leadwire 164 which comes from RF ablationequipment (not shown) which is external to the patient. The sensing ringelectrodes 16 and 18 are coupled to leadwires 12 and 14 which runthrough the center of the probe or catheter and also are connected toexternal equipment which is used to monitor electrical cardiac activity.These would typically be connected to an ECG or EKG recorder.

FIG. 56 shows a probe or catheter 10 similar to that illustrated in FIG.55 except that the energy dissipating surface EDS has been convoluted sothat its surface area has been increased. Such increasing of the EDSsurface area, which is in contact with fluids, such as body fluids, willincrease the amount of energy that is dissipated.

FIG. 57 is very similar to FIG. 56 except that instead of convolutions,fins 166 have been added. These fins 166 also increase the surface areaand increase the amount of energy or heat which is dissipated intosurrounding fluids and tissues.

FIG. 58 is similar to FIGS. 56 and 57 except that the energy dissipatingsurface (EDS) has its surface area increased through various processeswhich are more thoroughly described in connection with FIGS. 59 and 60.FIG. 59 is an enlarged, fragmented sectional view of the EDS surfacetaken from FIG. 58. The energy dissipating surface EDS area has beenroughened to create a high surface area, through, for example, plasmaetching 168, chemical etching, or the like. A high surface area can alsobe accomplished by porous coating deposits utilizing physical vapordeposition, chemical vapor deposition or electron beam depositionprocesses. Such porous coating deposits can include fractal coatings,metal nitrides, titanium nitrides, metal oxides, metal carbides, orvirtually anything that would provide a high surface or poroussubstrate. In addition, electrochemical deposition of porous coating,such as iridium-oxide, can also be utilized, as well as nucleate highsurface area morphologically structured coatings, such as whiskers,sub-micron filaments, tubes, nanotubes, or other morphologicalstructures such as columnar, titanium-nitride or iridium-oxide. Any ofthese types of surface conditionings can greatly increase the energydissipating surface area. FIG. 60, which is similar to FIG. 59,illustrates the use of carbon nanotubes or fractal coatings 170 toincrease the surface area and therefore the energy dissipation.

FIG. 61 shows a steerable catheter 172, which is typically used for avariety of applications including RF or cryo-ablation, cardiac mappingand many other purposes. Examples of RF ablation include treatment fornephrotic conditions, liver, brain, cancers and the like. For example,this would enable stereotactic ablation of certain lesions within thelung. An emerging field is the entire field of using ablation to treatvarious ventricular arrhythmias, including ventricular tachycardia. Theillustrated catheter 172 in FIG. 61 is meant to be representative of alltypes of catheters or probes which can be inserted into the venoussystem or other areas of the human body. The catheter 172 has a tip 174and an adjacent electrode surface 176, and a main catheter body 178,which can be steered around torturous paths. The steerable catheter 172has a handle 178 which can have various shapes, sizes and configurationsin the prior art. By twisting the illustrated cap 180 of the handle 178,one is able to steer the catheter 172 causing its tip 174 or othersegments to bend as one guides it.

FIG. 62 is an enlarged section taken along line 62-62 in FIG. 61. FIG.62 illustrates that the handle 178 includes an optional but preferredouter insulation sheath 182 which would typically be of plastic orsimilar material that would preferably not be highly thermallyconductive. Inside of the handle 178 are shown in cross-sectionleadwires 12 and 14. The illustration of two leadwires is not meant tobe limiting since any number of wires could be inside the handle 178 andcatheter 172 to sense electrical activity or deliver ablation energy. Inaccordance with the present invention, there are frequency selectiveimpedance elements 20 shown between the leadwires 12, 14 and an energydissipating surface EDS, such as a metallic sheath 184. The energydissipating surface EDS does not necessarily have to be metallic, but ithas to be capable of collecting RF energy and conducting thermal energy.This heat energy is therefore dissipated over the large surface area andthermal mass of the handle 178 itself. This results in very littletemperature rise, but at the same time, accomplishes the goal of thepresent invention in redirecting RF energy out of the leadwires 12 and14 that may be picked up by MRI RF pulsed fields and directing saidenergy into the relatively large surface area 184 inside the handle 178.Of course, one could eliminate the outer insulation sheath 182. However,in a preferred embodiment, the insulation sheath 182 would be relativelypoor in thermal conductivity so that one did really not feel anytemperature increase in his or her hand.

FIG. 63 is very similar to FIG. 55 except that a number of individual RFenergy or heat dissipating segments EDS₁, EDS₂ and EDS_(n) are shown.These are shown spaced apart by separation gaps d₁ and d_(n), which inreality can be quite small. The reason that these energy dissipatingsurfaces are segmented is so that they do not become physically andelectrically long enough to become a significant fraction or multiple ofa wavelength of the MRI pulsed frequency. Such short conductive sectionsdo not pick up significant energy from MRI whereas elongated leadwiresor conductors can, for example, resonate and pick up very significantamounts of MRI RF energy. It would be highly undesirable if the energydissipating surfaces, as illustrated in FIG. 63, were formed to becontinuous along the entire length of the catheter 10 as previouslydescribed in connection with FIG. 61. In this case, the energydissipating surface would actually become an energy collecting surfacebecause it would become a very effective antenna for the MRI pulsed RFsignals. Accordingly, breaking this up into discrete segments preventsthe EDS surfaces from actually becoming a receiver or antenna for theMRI induced energy.

FIG. 64 illustrates a paddle electrode 186 which could be used, forexample, in spinal cord simulator applications. It has eight electrodes188 housed in a biocompatible insulative and flexible body 190. Eightleadwires 192 are connected respectively to each of the eight electrodes188. As previously discussed, the elongated leadwires 192 can pick upsignificant amounts of RF energy during MRI scanning. It is veryimportant that the electrodes 188 do not overheat since they are indirect contact with the body, for example, with the spinal cord.

FIG. 65 illustrates the reverse side of the paddle electrode 186, wherean energy dissipating surface EDS is located. As shown in FIG. 66, onecan see that the electrodes 188 are conductive pads that contact thespinal nerve route or at least are closely associated with it. Theleadwires 192 are each electrically connected to respective electrodes188. There is a frequency variable impedance (or diverter) element 20 inaccordance with the present invention shown between each electrode 188and the energy dissipating surface EDS. These can individual discreetcapacitors or individual discreet L-C traps as shown in FIGS. 5 and 6.These can also be one continuous parasitic capacitance element thatformed between the overlap of each of the electrodes and the area of theEDS surface itself. In this case, the insulative dielectric material 194shown in FIG. 66 would be of relatively high dielectric constant. A highdielectric constant material is desirable so that the amount ofparasitic capacitance would be relatively large. By using parasiticcapacitance and appropriate dielectric materials, one eliminates theneed to use individually installed passive circuit elements. Referringto FIGS. 64-66, one can see that the undesirable RF energy is dissipatedon the opposite face of the paddle electrode 186 relative to theelectrodes that are in contact with the spinal nerve route. In otherwords, the RF or thermal energy is dissipated over a relatively largesurface area and is directed away from the sensitive juncture betweenthe electrode body tissue contact area. This is important for tworeasons, if the RF energy was allowed to concentrate on any one of theelectrodes due to resonance phenomenon, then a very high temperaturerise could occur which could cause thermal injury to the spinal nerveitself. By redirecting the energy in the opposite direction towards themuscle tissue and over a much larger surface area, much less temperaturerise occurs, and even if it does, it is directed into less sensitivetissue.

FIG. 67 illustrates a different type of paddle lead structure 196showing a total of fifteen electrodes 188. In this case there are twoenergy dissipating surfaces EDS and EDS′. For maximum surface area, theenergy dissipating surfaces could be on the top surface of the paddlelead structure 196, as well as on the backside or back surface (notshown). In accordance with the present invention, FIG. 68 illustrates afrequency selective variable impedance element 20 which is used todivert RF energy from the electrodes 188 to the EDS surfaces.

FIG. 69 is very similar to FIG. 28 in that it shows a section of humanhead with a deep brain stimulator disposed therein. There is a pluralityof leadwires 12 and 14 which are connected to an AIMD or pulse generator(not shown). The pulse generator would typically be placed in thepectoral region and leadwires 12 and 14 would be routed up along thepatient's neck to the deep brain electrodes 16 and 18. Referring toFIGS. 69-71, one can see that there is a novel tether 198 or wirearrangement where the leadwires 12, 14 are not only connected to thedistal electrodes 16, 18, but they are also connected to a pair ofenergy dissipating surfaces of EDS and EDS′. In FIG. 70, one can see thetether area 198 wherein the leadwires 12, 14 connect individually to theelectrodes. As shown in FIG. 71, the leadwires 12, 14 have a connectioninside the tether area 198 such that the wires are routed both to thedistal electrodes 16 and 18 and also through respective junctions 200and 200′ to two individual energy dissipating surfaces (EDS and EDS′).The leadwire 12 has a direct electrical connection at junction 200 todistal electrode 18. In turn, leadwire 14 has a direct connection atjunction 200′ to distal electrode 16. However, at the junctions 200 and200′, also connected are frequency selective elements 20 which in turnare connected respective energy dissipating pad or surfaces EDS andEDS′. Of course the separate energy dissipating pads could be one largeenergy dissipating pad. However, in order to maximize surface area andfacilitate surgical implantation, two pads are shown. These areoriginally implanted by the physician underneath a skin flap which isthen sewn back down in place. In this way, any heat that is generatedduring MRI procedures is generated on the top side of the skull wellaway from any brain matter.

It will be obvious to those skilled in the art that the presentinvention can be extended to a number of other types of implantablemedical devices, including deep brain stimulators, spinal cordstimulators, urinary incontinence stimulators and many other types ofdevices.

Although several embodiments of the invention have been described indetail, for purposes of illustration, various modifications of each maybe made without departing from the spirit and scope of the invention.Accordingly, the invention is not to be limited, except as by theappended claims.

1. A passive component network for an implantable leadwire of an activeimplantable medical device (AIMD), comprising: at least one leadwirehaving a length extending between and to a proximal end and atissue-stimulating or biological-sensing electrode at a distal tip end;and a frequency selective energy diversion circuit for diverting highfrequency energy away from the electrode to a point or an area spacedfrom the electrode for dissipation of said high-frequency energy,wherein the high frequency energy comprises an MRI frequency.
 2. Thepassive component network of claim 1, wherein said MRI frequencycomprises a range of MRI frequencies.
 3. The passive component networkof claim 1, wherein said MRI frequency in megahertz is selected from thegroup of frequencies comprising 42.56 times strength in Teslas of an MRIscanner.
 4. A passive component network for an implantable leadwire ofan active implantable medical device (AIMD), comprising: at least oneleadwire having a length extending between and to a proximal end and atissue-stimulating or biological-sensing electrode at a distal tip end;and a frequency selective energy diversion circuit for diverting highfrequency energy away from the electrode to a point or an area spacedfrom the electrode for dissipation of said high-frequency energy,wherein the frequency selective energy diversion circuit comprises a lowpass filter.
 5. A passive component network for an implantable leadwireof an active implantable medical device (AIMD), comprising: at least oneleadwire having a length extending between and to a proximal end and atissue-stimulating or biological-sensing electrode at a distal tip end;and a frequency selective energy diversion circuit for diverting highfrequency energy away from the electrode to a point or an area spacedfrom the electrode for dissipation of said high-frequency energy,wherein the frequency selective energy diversion circuit comprises atleast one series resonant LC trap filter.
 6. The passive componentnetwork of claims 4 or 5, including an energy dissipating surfacedisposed at a point or an area spaced from the electrode.
 7. The passivecomponent network of claim 6, wherein the energy dissipating surface isdisposed within the blood flow of a patient.
 8. The passive componentnetwork of claim 6, wherein the energy dissipating surface comprises aconductive housing.
 9. The passive component network of claim 6, whereinthe energy dissipating surface comprises a ring electrode.
 10. Thepassive component network of claim 6, wherein the energy dissipatingsurface comprises convolutions or fins, for increasing the surface areathereof.
 11. The passive component network of claim 6, wherein theenergy dissipating surface includes a roughened surface.
 12. The passivecomponent network of claim 11, wherein the roughened surface is formedthrough plasma or chemical etching, porous or fractal coatings orsurfaces, whiskers, morphologically designed columbar structures, vapor,electron beam or sputter deposition of a high surface area energyconductive material, or a carbon nanotubes.
 13. The passive componentnetwork of claim 8, wherein said diversion circuit is mounted withinsaid conductive housing.
 14. The passive component network of claim 9,wherein said conductive housing protects said diversion circuit fromdirect contact with patient body fluids.
 15. The passive componentnetwork of claim 13, wherein said conductive housing is hermeticallysealed.
 16. The passive component network of claim 6, wherein the energydissipating surface is disposed within an insulative sheath.
 17. Thepassive component network of claim 6, wherein the energy dissipatingsurface comprises at least a portion of a handle for a probe orcatheter.
 18. The passive component network of claim 17, wherein theenergy dissipating surface is disposed within an insulative sheath. 19.The passive component network of claim 6, wherein the energy dissipatingsurface comprises a plurality of spaced-apart energy dissipatingsurfaces.
 20. The passive component network of claim 6, including atether disposed between and conductively coupling the electrode and theenergy dissipating surface.
 21. The passive component network of claim6, wherein the energy dissipating surface comprises a material capableof being visualized during a magnetic resonance scan.
 22. The passivecomponent network of claims 4 or 5, further comprising a leadwirehousing supporting said leadwire distal tip electrode at one endthereof, said leadwire housing including a conductive housing portionforming an energy dissipating surface, and an insulator housing portionbetween said leadwire distal tip electrode and said conductive housingportion.
 23. The passive component network of claim 22, wherein saiddistal tip electrode cooperates with said conductive and insulatorhousing portions to define a hermetically sealed package having saiddiversion circuit mounted therein.
 24. The passive component network ofclaim 23, wherein said hermetically sealed package has a generallytubular shape with said distal tip electrode mounted at one end thereof,and further defining a hermetic seal assembly at an opposite endthereof, said at least one leadwire extending through said hermetic sealassembly.
 25. The passive component network of claim 23, furthercomprising an impeding circuit associated with said diversion circuitand mounted within said hermetically sealed package.
 26. The passivecomponent network of claim 25 wherein said impeding circuit comprises atleast one inductor.
 27. The passive component network of claim 26wherein said at least one inductor comprises an inductor chip.
 28. Thepassive component network of claim 23 wherein said diversion circuitcomprises at least one capacitor coupled between said leadwire and saidenergy dissipating surface.
 29. The passive component network of claim26, wherein said at least one inductor comprises a pair of inductorelements, said diversion circuit being coupled between said pair ofconductor elements and said energy dissipating surface.
 30. The passivecomponent network of claim 29 wherein said diversion circuit comprisesat least one capacitor.
 31. The passive component network of claims 4 or5, including an impeding circuit associated with the diversion circuit,for raising the high-frequency impedance of the leadwire, said impedingcircuit being disposed between said diversion circuit and the distal tipend of said at least one leadwire.
 32. The passive component network ofclaim 31, wherein the impeding circuit comprises an inductor.
 33. Thepassive component network of claim 31, wherein the impeding circuitcomprises a bandstop filter.
 34. The passive component network of claims4 or 5, wherein the at least one leadwire comprises a portion of a probeor a catheter.
 35. The passive component network of claim 34, whereinthe energy dissipating surface is selected from the group consistingessentially of a sheath, an insulative body, and a thermally conductiveelement.
 36. The passive component network of claims 4 or 5, whereinsaid at least one leadwire comprises at least a pair of leadwires eachhaving a length extending between and to a proximal end and atissue-stimulating or biological-sensing electrode at a distal tip end.37. The passive component network of claim 36, wherein said diversioncircuit couples each of said leadwires to an energy dissipating surfacedisposed at a point or an area distant from each of said electrodes. 38.The passive component network of claim 37, wherein said diversioncircuit is also coupled between said pair of leadwires.
 39. The passivecomponent network of claim 36, wherein said diversion circuit is coupledbetween said pair of leadwires.
 40. The passive component network ofclaims 4 or 5, further comprising an energy dissipating surface, andmeans for hermetically sealed mounting of said energy dissipatingsurface along said at least one leadwire between said proximal anddistal ends thereof, said hermetically sealed means defining a chamberhaving said diversion circuit mounted therein.
 41. The passive componentnetwork of claim 40, wherein said at least one leadwire comprises afirst leadwire having a tip electrode at a distal end thereof, and asecond leadwire having a ring electrode at a distal end thereof, saidfirst and second leadwires having said energy dissipating surfacemounted thereon in hermetically sealed relation therewith and extendingthrough said chamber, said diversion circuit coupling said first andsecond leadwires to said energy dissipating surface.
 42. The passivecomponent network of claim 41, wherein said diversion circuit comprisesa unipolar or multi-polar feedthrough capacitor.
 43. The passivecomponent network of claim 40, further comprising an impeding circuitassociated with said diversion circuit and mounted within saidhermetically sealed chamber.
 44. The passive component network of claims4 or 5, wherein the active implantable medical device (AIMD) comprises aprobe.
 45. The passive component network of claim 44, wherein saidelectrode comprises an ablation tip electrode at or near said leadwiredistal end.
 46. The passive component network of claim 45, furthercomprising a probe housing having said ablation tip electrode at or neara distal end thereof, and an energy dissipating surface carried by saidprobe housing at a point or an area distant from said ablation tipelectrode, said diversion circuit diverting high frequency energy awayfrom said ablation tip electrode to said energy dissipating surface. 47.The passive component network of claim 45, wherein said at least oneleadwire comprises a first leadwire having said ablation tip electrodeat or near a distal end thereof, and at least one second leadwire havinga ring electrode at or near a distal end thereof, and an energydissipating surface carried by said probe housing at a point or an areadistant from said ablation tip electrode and said ring electrode, saiddiversion circuit diverting high frequency energy away from saidablation tip and said ring electrodes to said energy dissipatingsurface.
 48. The passive component network of claim 4, wherein the lowpass filter comprises a capacitor, an inductor, a Pi filter, a T filter,an LL filter, or an “n” element filter.
 49. The passive componentnetwork of claim 5, wherein the frequency selective energy diversioncircuit comprises a plurality of LC trap filters resonant respectivelyat different MRI frequencies.
 50. A passive component network for animplantable leadwire of an active implantable medical device (AIMD),comprising: at least one leadwire having a length extending between andto a proximal end and a tissue-stimulating or biological-sensingelectrode at a distal tip end; a frequency selective energy diversioncircuit for diverting high frequency energy away from the electrode to apoint or an area spaced from the electrode for dissipation of saidhigh-frequency energy; and an energy dissipating surface disposed at apoint or an area spaced from the electrode; wherein the energydissipating surface comprises a conductive housing; wherein saiddiversion circuit is mounted within said conductive housing; and whereinsaid active implantable medical device (AIMD) comprises a deep brainstimulator.
 51. The passive component network of claim 50, wherein saidconductive housing is adapted for mounting in thermal communication witha patient's skull.
 52. The passive component network of claim 50,further comprising an electrode shaft assembly having a proximal endcarried by said conductive housing, said at least one leadwire extendingthrough said electrode shaft assembly and having said distal tip endelectrode for contacting patient brain tissue.
 53. A passive componentnetwork for an implantable leadwire of an active implantable medicaldevice (AIMD), comprising: at least one leadwire having a lengthextending between and to a proximal end and a tissue-stimulating orbiological-sensing electrode at a distal tip end; a frequency selectiveenergy diversion circuit for diverting high frequency energy away fromthe electrode to a point or an area spaced from the electrode fordissipation of said high-frequency energy; a leadwire housing supportingsaid leadwire distal tip electrode at one end thereof, said leadwirehousing including a conductive housing portion forming an energydissipating surface, and an insulator housing portion between saidleadwire distal tip electrode and said conductive housing portion,wherein said distal tip electrode cooperates with said conductive andinsulator housing portions to define a hermetically sealed packagehaving said diversion circuit mounted therein; and an impeding circuitassociated with said diversion circuit and mounted within saidhermetically sealed package; wherein said at least one inductorcomprises an inductor wire wound onto a ferromagnetic ornon-ferromagnetic mandrel.
 54. A passive component network for animplantable leadwire of an active implantable medical device (AIMD),comprising: at least one leadwire having a length extending between andto a proximal end and a tissue-stimulating or biological-sensingelectrode at a distal tip end; a frequency selective energy diversioncircuit for diverting high frequency energy away from the electrode to apoint or an area spaced from the electrode for dissipation of saidhigh-frequency energy; and a leadwire housing supporting said leadwiredistal tip electrode at one end thereof, said leadwire housing includinga conductive housing portion forming an energy dissipating surface, andan insulator housing portion between said leadwire distal tip electrodeand said conductive housing portion; wherein said distal tip electrodecooperates with said conductive and insulator housing portions to definea hermetically sealed package having said diversion circuit mountedtherein; and wherein said diversion circuit comprises at least one LCtrap filter coupled between said leadwire and said energy dissipatingsurface.
 55. A passive component network for an implantable leadwire ofan active implantable medical device (AIMD), comprising: at least oneleadwire having a length extending between and to a proximal end and atissue-stimulating or biological-sensing electrode at a distal tip end;a frequency selective energy diversion circuit for diverting highfrequency energy away from the electrode to a point or an area spacedfrom the electrode for dissipation of said high-frequency energy; aleadwire housing supporting said leadwire distal tip electrode at oneend thereof, said leadwire housing including a conductive housingportion forming an energy dissipating surface, and an insulator housingportion between said leadwire distal tip electrode and said conductivehousing portion, wherein said distal tip electrode cooperates with saidconductive and insulator housing portions to define a hermeticallysealed package having said diversion circuit mounted therein; and animpeding circuit associated with said diversion circuit and mountedwithin said hermetically sealed package, wherein said impeding circuitcomprises at least one inductor; wherein said at least one inductorcomprises a pair of inductor elements, said diversion circuit beingcoupled between said pair of inductor elements and said energydissipating surface; and wherein said diversion circuit comprises aplurality of LC trap filters resonant respectively at different MRIfrequencies.
 56. A passive component network for an implantable leadwireof an active implantable medical device (AIMD), comprising: at least oneleadwire having a length extending between and to a proximal end and atissue-stimulating or biological-sensing electrode at a distal tip end;a frequency selective energy diversion circuit for diverting highfrequency energy away from the electrode to a point or an area spacedfrom the electrode for dissipation of said high-frequency energy; and anenergy dissipating surface disposed at a point or an area spaced fromthe electrode; wherein the electrode comprises a paddle electrodedisposed on one side of a paddle.
 57. The passive component network ofclaim 56, wherein the energy dissipating surface is disposed on a secondside of the paddle.
 58. The passive component network of claim 57, wherethe frequency selective diversion circuit comprises a capacitive elementdisposed within the paddle between the electrode and the energydissipating surface.
 59. The passive component network of claim 58,wherein the capacitive element comprises a discrete capacitor.
 60. Thepassive component network of claim 58, wherein the capacitive elementcomprises parasitic capacitance.
 61. The passive component network ofclaim 56, wherein the energy dissipating surface is attached to thepaddle.